Adapted from Tender by Nigel Slater, via Food52
8 oz fresh beets
7 oz dark chocolate, broken into small pieces
1/4 cup hot, strong coffee
3/4 cup + 2 Tbsp butter, cut into very small pieces
1 cup + 2 Tbsp all-purpose flour
1 1/4 tsp baking powder
3 Tbsp cocoa powder
Scant 1 cup superfine sugar
Lightly butter an 8-inch springform cake pan and line the base with parchment. Pre-heat oven to 350 degrees.
Cook the beets, whole and unpeeled, in boiling unsalted water until tender - 30-40 minutes, depending on size. Drain and let cool in running water. Peel, slice off ends and run through blender or food processor until coarsely pureed.
Melt chocolate in a double-boiler or a bowl set over simmering water. Do not stir.
When the chocolate is almost melted, pour coffee over and stir once. Add butter and push down under surface of chocolate with a spoon and leave to soften.
Sift together flour, baking powder and cocoa.
Separate eggs. Place whites in large mixing bowl. Stir yolks together.
Remove chocolate from heat and gently stir until butter is melted. Let cool for a few minutes; stir in egg yolks. For each addition, add ingredients quickly and firmly, ensuring even mixing. Fold in beets.
Whisk egg whites until stiff and fold in sugar. Fold this into chocolate mixture. Finally, fold in dry ingredients.
Transfer batter to prepared cake pan and put into oven. Immediately decrease temperature to 325.
Bake for 40 minutes, or until only moist crumbs cling to a cake tester or toothpick.
Allow cake to cool. Do not try to remove from pan until completely cool