Start by preheating your oven to 180°C (350°F).
Grease your cake pan with butter and sprinkle it with flour; then set aside.
Using a medium-sized bowl, sift and mix flour, baking powder, and cocoa powder.
In a separate and larger bowl, combine sugar and butter, and mix until it becomes creamy.
Add eggs, milk, and vanilla extract, and continue mixing to combine.
Add the dry ingredients and mix.
Add 1/2 cup chopped walnuts and fold it into the butter cake batter.
Transfer the cake batter into the greased cake pan.
Use an angled spatula to even the cake and sprinkle the remaining walnuts on top of the cake.
Place the walnut butter cake pan in the middle of the oven for 40 to 45 minutes or until a toothpick comes out dry when you stick it in the cake.
Take the pan out of the oven and set it aside to cool.
Slice your chocolate walnut butter cake and serve.