Heat oven to 160°C.

Line a square cake tin with butter and baking paper.

Roughly chop your Dr. Oetker Dark Chocolate.

Set a heatproof bowl over a saucepan filled with boiling water. Add your chocolate and butter, then gently melt. Set aside to cool slightly.

Add your instant espresso powder to a bowl, then pour in your coffee liqueur and 1 tbsp boiling water. Give it a mix, then set aside to cool.

Add your eggs and sugar to a bowl and whisk them with an electric beater until light and mousse-y.

Gently fold in the chocolate mixture until it is mostly combined.

Sift in your plain flour, cocoa powder and baking powder. Gently fold it in. Add your cooled coffee and vanilla extract and give it one last mix.

Add your mascarpone to a bowl along with your remaining egg yolk, 2 tbsp sugar and your remaining coffee liqueur. Whisk to combine.

Pour your batter into your prepared baking tin. Pour your mascarpone mixture over the top, then swirl slightly with a palette knife. Bake for 30 mins.

Allow to cool totally, then cut into squares and serve.