Preheat the oven to 170°C. Prepare a small sheet pan with a baking paper on it.
Heat the butter in the heatproof bowl in the microwave for 20 seconds (or on the stove until warm), and add the sugars. Mix well with an egg whisker for 2 minutes and repeat the process - heat up and mix.
Chop up the chocolate and add to the warm mixture. Mix well until the chocolate melts completely.
Add eggs and egg yolk and mix.
In another bowl prepare the dry ingredients - flour, baking powder, baking soda, cocoa powder, and a pinch of salt. Add them to the chocolate mixture. Mix until flour disappears.
Pour the brownie mix into the prepared sheet pan. Melt the white chocolate and pour it over the mix. Bake for 20-30 minutes.
As soon as it is 8 minutes until the end of baking, take out the sheet pan and put marshmallows on top. Bake for 8 minutes again.
Take out the brownies and let cool completely before cutting.