Preheat the oven.top/bottom heat: about 200°hot air: about 180°CFor the dough, mix the flour and baking powder and place in a mixing bowl. Add remaining ingredients and mix well with hand mixer with dough hook, first briefly on lowest, then on highest speed.Then knead the dough briefly on a lightly floured work surface. If it sticks, wrap it in plastic wrap and chill it for a while.Roll out the dough on a baking sheet (30x40cm, greased) to a square (about 30x30cm) about 1/2cm thick and prick it several times with a fork. Put the baking sheet in the preheated oven and bake the base for 15-20 minutes.Let the base cool on the baking sheet on a cooling rack.For the buttercream, prepare a pudding from pudding powder, sugar and milk according to package instructions. Allow pudding to cool, stirring occasionally (do not refrigerate).Melt butter and coconut oil, allow to cool slightly and add to pudding while stirring. Mix the cream well and spread on the cooled base.For the topping, toast the walnut kernels with butter and sugar and spread on the cream while still warm. Melt the couverture in a small saucepan in a bain-marie over low heat, stirring. Place in a freezer bag and cut off a small corner. Sprinkle cake with the couverture.