For the dough: Place 120 g softened butter, 50 g icing sugar, 50 g ground almonds, and 1 egg yolk in a food processor. Blend until smooth. Add 150 g flour all at once and mix to form a dough ball. Freeze for 10 minutes to firm up.
Preheat the oven to 210°C (gas mark 7).
Roll out the dough on a lightly floured surface and line a buttered tart pan with parchment paper. Prick the base with a fork, top with another sheet of parchment, and weigh down with dried beans or baking stones.
Cut leaf shapes from the dough scraps and attach around the tart edge, moistening slightly to stick. Dust with cocoa powder and blind bake for about 20 minutes.
Meanwhile, break the 250 g dark chocolate into pieces and melt with 2 tablespoons water.
Bring the 30 cl whipping cream to a boil, then pour gently over the melted chocolate, stirring until smooth. Add the butter and mix well for a glossy ganache.
Prepare a light blond caramel by heating 100 g sugar with 6 tablespoons water, then pour over the 200 g walnut kernels. Reserve 3 caramelized walnuts for decoration. Crush the rest into a coarse powder in the food processor.
Unmold the baked tart shell onto a serving dish, discard the parchment, spread the walnut powder over the base, pour on the chocolate ganache, and refrigerate for at least 2 hours.
To serve, garnish with chocolate shavings and the reserved caramelized walnuts.
