This recipe appeared in this month's Good Food. While I would have substituted granulated sugar for caster sugar and brown sugar for muscovado, my auntie found both sugars and quark at Whole Foods.
For the cake
1 tsp instant coffee, mixed with 1 tbs warm water (I just used a tbs of coffee leftover from the morning)
4 eggs, room temp
85 g/3 oz golden caster sugar, plus extra for rolling
50 g/2 oz light muscovado sugar
120g/4.5 oz self-raising flour
2 tbs cocoa powder
For the icing
50g/2 oz 70% dark chocolate (I doubled this), chopped
250 g/9 oz mascarpone light
140g/5 oz quark (This is best described as a sour cream cheese--I did not like its strong flavor and will use regular cream cheese in the future)
50g/2 oz powdered sugar
1.5 tbs cocoa powder
1 tsp instant coffee
1/2 tsp vanilla
Heat the oven to 375 degrees. Line with parchment paper and lightly spray a 15x9 jelly roll pan.
Beat the eggs, both sugars, and the coffee together with an electric whisk for about 8 minutes until it is thick and airy and hold a trail when you lift the beaters. (This process amazed me. The mixture also thickened as it stood, so it may be okay to only beat for 6 minutes and let it stand while you prepare the dry ingredients).
Sift flour and cocoa powder together and gently fold into egg mixture. Pour into prepared baking pan, spreading gently to make it even. Bake for 12 minutes or so until it feels spongy but firm when touched.
Dust another sheet of baking paper with caster sugar and turn out cake. (This must be done soon after taking it out of the oven). Peel away the old paper. Roll the cake, keeping the new paper inside into a log. Cool completely or put in the fridge overnight, which aids moistness.
Prepare the icing by first melting the chopped chocolate in a double boiler. Cool.
Beat the mascarpone and quark (use cream cheese) together and sift in the powdered sugar and cocoa. Stir in about 2/3rds of the melted chocolate, coffee, and vanilla. (Reserve the rest of the melted chocolate to drizzle over the top).
Unroll the cake and discard the paper. Spread half of the icing evening over the cake and reroll, putting the seam on the bottom. Spread the rest all around the top and sides of the log, smoothing in any cracks. Drizzle with remaining chocolate and dust with powdered sugar, if desired.