Place the chopped butter and chocolate in a small bowl and melt gently over a pot of boiling water for 5 minutes, before giving it a stir to combine. Remove from the cooktop when mixture is melted and glossy.
Add sugar to the chocolate mixture and stir until well combined.
Add the eggs, one at a time, beating well after each addition.
Stir in vanilla essence, and then gently fold in sifted flour and cocoa
Stir in walnuts (after crushing or chopping down to smaller pieces)
Place patty cases in muffin tins or butter the tins very well
3/4 fill each muffin tray patty case with the chocolate mixture (hint; use two spoons to put the mix in the cases).
Bake for 8 minutes, and then rotate the trays, bake for 7 more minutes and test - they should be slightly cracked on top. Remove from the oven and leave to cool or place in the fridge.