In a saucepan, boil together 125ml water, 125ml milk, 112g butter, 6g salt and 6g sugar.
Add 137g flour and stir with a spatula.
Add 220g eggs one at a time and mix well.
Use a piping bag to make the little choux mounds on a baking tray covered with parchment paper. Sprinkle the buns with icing sugar.
Bake for 20 minutes in a 160C oven. Slice off the top third of each bun. Whip together 100g mascarpone, 300ml whipping cream and 40g sugar to make Chantilly cream and pipe the cream into the choux buns, then replace the tops.