Follow these detailed steps carefully for flawless raspberry chocolate tartlets. We will work in three stages: preparing the shells, making the ganache, and finally, assembling the dessert.

Part 1: Crafting the Perfect Chocolate Tartlet Shells

1. 1. Combine Dry Ingredients: In a large mixing bowl or the bowl of a food processor, whisk together the all-purpose flour, unsweetened cocoa powder, granulated sugar, and salt. This ensures all the dry components are evenly distributed before adding the fat.
   2. Cut in the Cold Butter: Add the cold, cubed butter to the flour mixture. Using a pastry blender, two knives, or the pulse function on your food processor, cut the butter into the flour until the mixture resembles coarse, sandy crumbs with some pea-sized pieces of butter still visible. It is crucial not to overmix here; those visible butter pieces are what create the flaky texture.
   3. Add the Wet Ingredients: In a small bowl, whisk together the lightly beaten egg and vanilla extract. Drizzle this mixture over the flour and butter crumbs. Mix with a fork or pulse in the food processor just until the dough starts to come together in large clumps. Do not knead it like bread dough. Overworking the gluten will result in tough, chewy shells instead of crisp, tender ones.
   4. Form and Chill the Dough: Turn the dough out onto a clean, lightly floured surface. Gently press and gather it into a cohesive disc. It might seem crumbly at first, but it will come together with gentle pressure. Wrap the disc tightly in plastic wrap and refrigerate for at least 1 hour, or up to 2 days. Chilling the dough is a non-negotiable step; it allows the gluten to relax (preventing shrinkage) and firms up the butter (ensuring flakiness).
   5. Prepare for Baking: Preheat your oven to 375°F (190°C). Lightly grease and flour six 4-inch tartlet pans with removable bottoms. This will ensure your delicate shells release easily after baking.
   6. Roll and Shape the Dough: Remove the chilled dough from the refrigerator. If it’s very firm, let it sit at room temperature for 5-10 minutes to become more pliable. On a lightly floured surface, roll the dough out to about ⅛-inch thickness. Use a cookie cutter or a small knife to cut circles slightly larger than your tartlet pans. Gently press one circle of dough into each pan, making sure it fits snugly against the bottom and up the sides. Trim any excess dough from the rim with a sharp knife or by rolling your rolling pin over the top edge of the pan.
   7. Dock and Freeze: Use the tines of a fork to gently prick the bottom of each tartlet shell a few times. This process, known as “docking,” allows steam to escape during baking, preventing the crust from puffing up. Place the prepared shells in the freezer for 15-20 minutes to firm them up again. This final chill is another key step to minimize shrinkage in the oven.
   8. Blind Bake the Shells: Line each frozen shell with a small piece of parchment paper or a cupcake liner and fill with pie weights, dried beans, or uncooked rice. This is called blind baking, and it ensures the crust bakes evenly and holds its shape. Bake for 15 minutes.
   9. Finish Baking: Carefully remove the parchment and weights. Return the shells to the oven and bake for another 8-10 minutes, or until the shells look dry, firm, and are fragrant. Let them cool in their pans on a wire rack for 10 minutes before carefully removing the outer rings. Allow the shells to cool completely to room temperature before filling.

Part 2: Making the Silky Chocolate Ganache

1. Prepare the Chocolate: Place your finely chopped dark or semisweet chocolate in a medium-sized, heatproof bowl. The finer the chocolate is chopped, the faster and more evenly it will melt.
2. Heat the Cream: In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer around the edges. You’ll see small bubbles forming. Do not let it come to a rolling boil, as this can cause the ganache to split or become greasy.
3. Combine and Rest: Pour the hot cream directly over the chopped chocolate. If using, add the optional espresso powder to the cream. Let the mixture sit, undisturbed, for 5 minutes. This allows the heat from the cream to gently and thoroughly melt the chocolate without scorching it.
4. Emulsify the Ganache: Starting from the center of the bowl, begin whisking the mixture slowly and gently. As you whisk, you will see the chocolate and cream start to combine, forming a smooth, glossy emulsion. Continue whisking, moving outwards in a circular motion, until the ganache is completely smooth and uniform.
5. Add Finishing Touches: Add the room temperature butter and the optional corn syrup. Continue to whisk gently until the butter is fully incorporated and the ganache is exceptionally shiny and smooth.
6. Cool the Ganache: Let the ganache cool at room temperature for about 20-30 minutes, or until it has thickened slightly to a pourable, pudding-like consistency. It should still be fluid but not hot. Pouring hot ganache into the shells can make them soggy.

Part 3: Assembly and Finishing Touches

1. Fill the Shells: Place your completely cooled chocolate tartlet shells on a baking sheet for easy transport. Carefully pour or spoon the slightly cooled ganache into each shell, filling them just to the top edge.
2. Set the Ganache: Transfer the filled tartlets to the refrigerator and chill for at least 2-3 hours, or until the ganache is firm to the touch. This step is crucial for a clean slice and a satisfying texture.
3. Arrange the Raspberries: Just before serving, arrange the fresh raspberries on top of the set ganache. You can place them in a neat circle, pile them in the center, or create any pattern you like.
4. Serve and Enjoy: Serve the tartlets chilled or at a cool room temperature for the best flavor and texture.