Preheat oven to 350°F.

Butter and dust a springform pan with cocoa powder. Line the bottom with a greased disc of parchment paper.

In a large pyrex or microwave-safe bowl, melt butter (1-2 minutes)

Add chocolate, turbinado sugar, and mascarpone to melted butter. Stir to combine.

In a large bowl, sift together flour, cornstarch, cocoa powder, salt, and espresso powder.

In a stand mixer, whip egg whites for 1-2 minutes until foamy. Slowly add white sugar and beat on high until a stiff meringue forms.

Fold the sifted dry ingredients into your melted chocolate and butter mixture. You will have a stiff brownie-like batter.

Fold in your stiff egg whites one spoonful at a time until no streaks remain. Be careful not to deflate the air in the egg whites.

Once your batter has the consistency of chocolate mousse, transfer to prepared springform pan and bake for 30 minutes.

Cool and remove from pan.