Heat oven to 350°F (175°C).
Grease or line your baking pan.
In a bowl, mix butter and sugar.
Add eggs and vanilla. Stir well.
Add cocoa, flour, and salt. Mix again.
Pour this brownie mix into the pan.
In another bowl, mix mascarpone and sugar.
Add yolk, vanilla, and coffee. Mix until smooth.
Spoon this mix over the brownie layer.
Swirl the top gently with a knife.
Bake for 30 to 35 minutes.
Let brownies cool in the pan.
Dust with cocoa powder on top.
Cut into squares and enjoy!