For the shortcrust pastry: Preheat the oven to 325 degrees F. Combine the butter and confectioners' sugar in a bowl, whisking together until creamy and pale yellow. Add the egg yolks and mix until combined and fluffy.
Add in the flour, ground cashews, cocoa powder and salt and mix together with a wooden spoon until combined.
As soon as the dough comes together, shape it into a square. Place the dough over a lightly-floured work surface and sprinkle some flour on top. Roll out the dough into a 13-by-9-inch rectangle 1/2-inch thick.
Roll the dough over the rolling pin to transfer it to an 11-by-8-inch tart pan. Ease the dough into the bottom and corners of the pan gently, then roll the rolling pin over the top of the pan to cut off any excess dough. Poke some holes all over with a fork.
Place the crust in the freezer for 10 minutes (this will prevent shrinkage when it is baked).
Bake the crust until it looks chalky and very opaque, 15 to 20 minutes. Set on a rack to cool.
For the bittersweet chocolate ganache: Melt the milk chocolate slowly in the top of a double boiler or a heatproof bowl set over simmering water. Remove from the heat and let cool slightly.
Combine the cream and honey in a small saucepan and bring to a boil; turn off the heat. Gradually pour the cream-honey mixture into the melted chocolate. Using a flexible spatula, mix it in energetically, drawing small circles with the spatula for an elastic, shiny consistency. Stir in the butter until shiny and well-combined.
Brush a fine layer of melted chocolate over the baked tart crust to seal it and prevent it from getting soggy. Let cool until completely hardened.
Pour the ganache into the crust and chill for about 2 hours. Bring the tart to room temperature before serving.