Preheat oven to 175°C (350°F) and line a 9x9-inch pan with parchment paper.Whisk melted butter, granulated sugar, and brown sugar until smooth and glossy.Add eggs and vanilla, whisking until fully combined.Fold in cocoa powder, flour, and salt until just combined.Spread batter evenly in pan and bake for 22–26 minutes.Cool brownies completely in the pan.Whip cream with powdered sugar until soft peaks form.Fold whipped cream into mascarpone until smooth.Spread topping over cooled brownies, chill briefly, slice, and serve.