For the chocolate sheet cake, preheat the oven to 160 °C (hot air). Cut butter into pieces, break chocolate into pieces and liquefy both together in a pot or in steam while stirring.
Whisk eggs for a few minutes, add sugar and add the now cooled chocolate-butter mixture.
Stir in almonds, flour and cinnamon. Line baking sheet with parchment paper, spread batter on it and bake for fifteen minutes. Serve sprinkled with powdered sugar.