Take the butter, sugar and egg yolk and whisk it.
Add flour and milk to the dough a little bit at a time, whisking it all.
Add the egg white (already whisked) and than you can add the baking powder slowly mixing all together.
Now the dough is complete!
Fill in the baking cup with a little bit of dough (1/3).
I've used nutella and mulberry jam as filling, but you can choose yours. I like to put filling in the freezer for some time to make it a little bit harder.
Place it in the middle of the baking cup, be cautious not to let it stick at the edges.
Fill in the rest of the baking cup, but leave a 1/3 of the recipient free to let the cake grow. Place in a preheated oven for 20 minutes and...
Voilà! Just feel how good they taste!