Combine the flour, confectioners’ sugar, and salt in a food processor, then pulse briefly to distribute them. Add the cold butter and pulse until the mixture resembles coarse sand with a few pea-size pieces remaining. Add the egg yolk and cold cream, then pulse just until moist clumps hold together when pressed. Do not process the dough into a smooth ball, since excess mixing develops gluten. Turn it onto the counter, press it into a 1-inch-thick disk, and wrap tightly.
Refrigerate the wrapped dough for 60 minutes, until firm but still flexible. Set a 9-inch removable-bottom tart pan on the counter. On a lightly floured surface, roll the dough into an 11-inch round about 1/8 inch thick, turning it frequently to prevent sticking. Ease it into the pan without stretching, press it into the corners, and trim the rim flush. Patch any cracks with scraps, dock the base lightly with a fork, and freeze the lined pan for 20 minutes.
While the shell freezes, position a rack in the lower third of the oven and preheat to 375°F or 190°C. Place the tart pan on a rimmed baking sheet for stable handling. Crumple a sheet of parchment, flatten it, and fit it closely against the frozen pastry. Fill the parchment to the rim with pie weights or dry beans, making sure the weights support the sides. This firm contact limits slumping and prevents the base from ballooning during the first bake.
Bake the weighted shell for 18 minutes, until the upper edge looks dry and lightly golden. Carefully lift out the parchment and weights, then return the uncovered shell to the oven for 10 to 12 minutes. The base should appear matte, evenly golden, and firm rather than pale or greasy. If it puffs, press it down gently with the back of a spoon. Transfer the pan, still on its baking sheet, to a wire rack and cool completely for at least 30 minutes.
Whisk the granulated sugar and cornstarch together in a medium saucepan before adding the 300 g raspberries and lemon juice. Cook over medium heat, stirring and crushing the berries with a silicone spatula, until the mixture releases juice and begins bubbling throughout, about 4 to 6 minutes. Continue cooking for 1 full minute; the spatula should leave a trail that closes slowly. Scrape the filling into a shallow bowl, cover the surface, and cool until no longer warm, about 30 minutes.
Spread the cooled raspberry filling evenly across the fully cooled shell with an offset spatula. Place the chopped chocolate in a dry heatproof bowl. Heat the 220 g cream in a clean saucepan over medium heat until steaming with small bubbles around the edge, then pour it over the chocolate. Rest for 2 minutes before stirring slowly from the center outward until glossy. Blend in the room-temperature butter. Let the ganache cool for 5 minutes, then pour it gently over the raspberry layer and level the surface.
Refrigerate the assembled tart uncovered until the ganache is firm at the edges and set in the center, at least 3 hours. Arrange the 125 g fresh raspberries over the surface only after it has set, pressing very lightly so they stay in place. To unmold, set the pan on a sturdy can and lower the outer ring, then slide the tart onto a serving plate. Rest it at room temperature for 20 minutes. Slice with a warm, dry knife, wiping the blade between cuts.
