Preheat your oven to 350°F (180°C) and prepare a 10x15 inch jelly roll pan by greasing it and lining it with parchment paper.

In a mixing bowl, sift together flour, Dutch-processed cocoa powder, baking powder, salt, and instant coffee.

In a separate bowl, whisk the egg yolks with half of the caster sugar until the mixture becomes thick and pale in color, about 3-4 minutes.

Using a clean mixing bowl, beat the egg whites until they become frothy. Gradually add in the remaining sugar and continue to whip until stiff peaks form, which should take about 4-5 minutes.

Gently fold one-third of the whipped egg whites into the yolk mixture to lighten it. Then, carefully add the remaining meringue and sifted dry ingredients in two parts, folding gently.

Pour the batter into your prepared jelly roll pan, spreading it evenly to the corners. Bake for 12-15 minutes or until the cake springs back when lightly touched.

Once baked, immediately turn the cake out onto a Cocoa dusted kitchen towel. Peel off the parchment paper and roll it up tightly with the towel to shape it.

In a cold mixing bowl, whip the heavy cream, powdered sugar, and vanilla until soft peaks form. Then, gently fold in the mascarpone cheese and beat until the mixture thickens.

Carefully unroll the cooled cake and spread the whipped filling evenly over the surface. Roll it back up without the towel.

In a saucepan over low heat, melt the dark chocolate, butter, brown sugar, and heavy cream together.

Once chilled and set, unwrap the Swiss roll and carefully place it on a serving platter. Drizzle the cooled chocolate sauce generously over the top and cut into slices.