You'll need accurate scales, a small saucepan, a mixing bowl and a fork. Pre-heat oven to 180-190c (conventional/non-fan). Get two six-hole muffin tins or a twelve-hole tin and pop paper cases in each hole. The cases need support or the mix will spread out as it bakes. This recipe makes exactly 12 muffins in the Sainsbury's cases I use (shown in pic).

Dry ingredients in a mixing bowl:

250g Self Raising flour
1 teaspoon of baking powder
150g sugar
Scant half teaspoon of salt
Two heaped tablespoons of cocoa sifted into the rest of the dry ingredients
Mix together with a fork.

Wet ingredients:

Pop a saucepan on your scales and weigh in 90g of unsalted butter. Melt it over a very low heat. Let it cool enough for you to put the pan back on the scales. Add 150g cold milk. Stir. Add one medium egg and one teaspoon of vanilla extract. Whisk it all together with the fork.

Pour the wet ingredients straight over the dry ingredients in the mixing bowl and start mixing with your fork. Don't overmix. It should take you less than a minute. If there are small lumps, that's fine. If there are huge islands of raw flour, that's not fine. Divide equally between the paper cases and put straight in the oven. They'll take about 10-12 minutes. Take the paper cases out of the tin as soon as you can and let the muffins cool on a wire rack.