Preheat oven to 350°F (175°C) and line an 8 or 9‑inch square baking pan with parchment paper.
Melt the chopped dark chocolate and butter together in a microwave‑safe bowl in 30‑second intervals, stirring between each, until smooth.
Add both sugars and vanilla extract to the melted mixture and stir well.
Crack in the eggs one at a time, whisking fully after each addition.
Sift in the flour and salt and gently fold until just combined.
Pour the brownie batter into the prepared pan and smooth the top with a spatula.
Bake for 25 to 30 minutes until the edges are set and the centre is slightly soft.
Remove from the oven and while still warm use a skewer or fork to poke holes across the brownie surface. Pour the espresso shots over the warm brownies and let cool completely.
In a clean bowl whisk the mascarpone cheese, heavy cream and vanilla extract until smooth.
Add the sugar and espresso powder and whip until the mixture holds soft peaks. Spread over the cooled brownies and smooth the top.
Dust generously with cocoa powder using a fine sieve, then chill briefly if desired before slicing into squares.