Preheat oven to 170C.
Grease pan, and line bottom with parchment paper.
Cream butter and sugar together until fluffy.
Add eggs and beat one at a time.
Beat on high for 1 minute to incorporate air into batter.
Add milk & vanilla extract and beat until combined.
Sift in flour, cocoa powder, and baking powder.
Beat until just combined.
Fold in milled/blended walnuts.
Pour into baking tin, drop tin on counter tops a few times to remove large air bubbles.
Bake at 170C for 50-55 minutes for 9 inch cake, or 40 minutes for 6 inch cake.
Check by inserting skewer into cake, if it comes out clean, it means it's done.