Preheat the oven to 180°C (approximately 350°F) top and bottom heat.
Coarsely chop the chocolate and melt over a hot, not boiling water bath.
Beat the butter with the sugar until creamy, stir in the eggs and gradually add the slightly cooled chocolate. Mix in the flour and stir in the nuts.
Spread the mixture on a parchment paper-lined baking sheet and bake in the preheated oven for about 30 minutes.
Remove from the oven and let cool. Cut into pieces, sprinkle with powdered sugar and garnish with pink marzipan hearts, if desired. Serve.