[Preparation] Add the butter to a bowl and leave to warm to room temperature. Sift the dry ingredients marked ◎. Bring the egg to room temperature as well. Preheat the oven to 180℃.
Cream the butter and add the sugar in 2-3 batches, mixing well after each addition. Add salt and keep mixing until it becomes soft and light.
Add a well beaten egg in 2-3 batches. If you blend the butter and sugar properly and add the egg a little at a time whilst continuing to mix, the mixture should look like brown sugar butter cream, as in the photo.
Sift in the dry ingredients marked ◎ in 2 - 3 separate batches and gently fold into the mixture with a rubber spatula.
Pour the mixture into muffin cups 80% full and top with whatever you like, such as nuts, etc. Bake in a preheated oven for 15 minutes. You may need to adjust the baking time slightly depending on your oven and muffin cups.
Check to see if they're done by inserting a skewer into the center of the muffin. If no raw batter sticks to the skewer, then they're ready. This is optional, but if you like they'll taste amazing brushed with rum whilst still hot. Leave to cool before wrapping with cling film and chilling further.
[Basic Variation] I mixed raisins into these muffins. For 3 muffins, use about 10 g raisins. Add the raisins to the sifted ingredients in Step 4 and fold into the mixture. If you add the raisins after the sifted ingredients they won't sink to the bottom.
[Brown Sugar Coffee Version] Mix the ingredients marked ☆ together and mix into the main batter after sifting in 1/4 of the dry ingredients. You should mix here and fold in afterwards. The rest of the recipe is the same as written above.
[Brown Sugar Coffee Version Extra] I think peanuts go really well as a topping for this. Shredded coconut also goes well but you can add what you like.
[Comparing molds] I tried baking these in 6 x 2 cm madeleine cups as well. You can make about 10 madeleines. I baked them for 10 minutes but as they're made of paper and quite thin, so I did have to adjust the baking time slightly. The brown sugar coffee version should make around the same amount of cupcakes too.