Pre-heat your oven to 180C. Line tray with muffin cups. Sift and mix dry ingredients together; flour, cocoa powder, baking powder and sugar.
Mix eggs, milk, vanilla extract and melted butter together.
I had some odd and ends on dark chocolate in my fridge, I just took a pounder and whacked the daylights out of them to get 200 gms chunks.
Pour half the chocolate chunks into the dry mixture.
Unwrap the peanut butter cups and place one on each muffin cup. Better to do this while they're hard and cold, if not they're rather messy to deal with.
Mix the wet ingredients with the dry ingredients. Fold them gently until all the flour mixture has been incorporated. If there's lumps, don't worry, some lumps is fine.
Fill the muffin cups (with Reese's inside) with the batter till about 3/4 full.
Sprinkle the rest of the chocolate chunks on top.
Bake for about 30 mins, it's hard to see if they're browning on top because of the chocolate colour so use the toothpick test. If you get chocolate stains on the toothpick but not the batter, it's done. The tops should be slightly hardened and crisp but inside still moist and fluffy.
Cool muffins on a rack before eating.
They're best eaten warm so if you freeze them, pop them in the microwave under medium heat for 1 min and they're good as new.