Preheat the oven to 180 C / Gas 4. Line 2 (12-hole) muffin tins with paper cases.
Beat the butter in a mixing bowl to soften then add the sugar, flour, salt, eggs and milk and beat until smooth. Add the cocoa powder at this stage if you want to make chocolate muffins. Spoon the mixture into the prepared paper cases, filling to 3/4 full to ensure a good rise.
Bake in the preheated oven for 15 minutes, then swap them around in the oven and continue baking for a further 3 to 7 minutes, or until risen and golden and a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool before serving.