"It's so rich, moist, and delicious, and so ridiculously easy to make! I always bake this for birthdays; it's the cake that gets the most requests among my chocolate-loving family and friends. To serve, sprinkle generously with almond slivers and drizzle with melted chocolate."
Combine 3/4 cup plus 1 tablespoon chocolate-hazelnut spread, dark chocolate, and 1/2 cup plus 1 tablespoon butter in a heat-proof bowl set over a pan of hot water; stir until melted, 3 to 5 minutes. Allow to cool slightly.
Whisk egg yolks and brown sugar together in a bowl. Add to melted chocolate mixture; mix in ground almonds.
Whisk egg whites together in a bowl. Add 1/3 to the chocolate-almond mixture and stir. Fold in the remaining 2/3. Pour into a cake pan.
Bake in the preheated oven until set, 50 to 60 minutes.