Preheat the oven to 180°C (356°F).
Mix the butter and sugar together until creamy.
Add the egg and vanilla extract and mix well.
Sift in the flour, cocoa powder, baking powder, and salt, then mix until combined.
Add chocolate chips and walnuts, and mix evenly.
Shape the dough into cookies and place them on a baking pan.
Bake in the oven at 180°C for 12~15 minutes.
Once the cookies cool, layer with chocolate cream and whipped cream between each layer.