Preheat the oven to 350°F (180°C). Grease and line a 10×15 inch jelly roll pan with parchment paper.

In a medium bowl, sift together the all-purpose flour, cocoa powder, baking powder, salt, and instant coffee. Whisk to combine.

In a large bowl, whisk the egg yolks with half of the caster sugar until thick and pale, then fold in melted butter and vanilla extract.

In a clean bowl, beat the egg whites until frothy, gradually adding remaining caster sugar until stiff peaks form.

Fold one-third of the egg whites into the yolk mixture, then gently fold in remaining whites and sifted dry ingredients.

Pour the batter into the prepared pan, spreading evenly. Bake for 12-15 minutes until it springs back when pressed.

Invert the pan onto a clean, cocoa-dusted kitchen towel, remove parchment, and roll the cake tightly while warm. Let cool completely in this position.

Whip the heavy cream with powdered sugar and vanilla until soft peaks form, then gently fold in mascarpone cheese.

Once the sponge is cool, unroll it and spread the Chantilly cream filling evenly, then roll back up without the towel and refrigerate for at least 30 minutes.

Melt dark chocolate, butter, brown sugar, and heavy cream in a saucepan over low heat, stirring until smooth. Cool before drizzling over the chilled roll.