There are thousands of recipes for cupcakes around, all different, and most of them say that the recipe should be followed exactly and all weights should be precise.
I've found that this is not true.
I can now eyeball this basic recipe by taking visually equal amounts of butter and sugar and adding flour and milk until it seems right. Also, up to half of the flour can be replaced with finely ground nuts, and any tasty-seeming flavouring is possible (for instance hazelnut and cocoa, known in this household as "Nutella cakes" because they taste exactly like it). You can also replace part or all of the milk with fruit juice, add raisins or other dried fruit (perhaps adding a little more liquid), use margarine instead of butter, or put a dollop of jam in the centre of the cakes before baking. I recently made almond cakes with a bit of rose-water and a rose-petal jelly centre: Arabian Nights exotic and delicious!
The butter should be soft to start with. If your butter is cold and hard, use a microwave-proof mixing bowl and microwave the butter in the bowl for 1 minute on the lowest setting.
This amount of batter makes 12 large (in muffin tins) or 18 small cupcakes.
Cream the butter and the sugar together with a wooden spoon. Add the egg and any flavouring agent that can't be sifted with the flour and mix (an electric beater is easier at this stage). Sift the flour with baking powder, salt and any powdery flavouring (like cocoa powder) and mix it in. Add enough milk, if necessary between portions of flour, to get a very thick batter or a very soft dough.
For easy shaping, put paper cupcake cases in the compartments of a muffin mould. If you're making lots of small cupcakes, or don't have a muffin mould, use two paper cases per cupcake. Fill them about 2/3 full. If you're putting jam in the centre, fill in two layers with the jam in between.
Bake at 175°C for 25-30 minutes or until a skewer stuck in the centre comes out clean. Cool on a rack.