Preheat the oven to 180°C (160°C fan, gas 4). Line the base of a springform cake tin with baking paper and grease the sides with butter.
Break the chocolate into pieces and melt in a heatproof bowl set over a pan of hot water. Add the butter in pieces and melt into the hot chocolate. Remove the bowl from the pan and allow the chocolate to cool slightly.
Mix the eggs in a bowl with the sugar and a pinch of salt until thick and creamy. Stir in the almonds, cinnamon and baking powder.
Slowly add the slightly cooled chocolate mixture.
Mix everything well with a spatula.
Pour the mixture into the tin and bake in the oven for 40 minutes.
Test if the cake is ready by inserting a cocktail stick into the centre. If it comes out clean, the cake is ready.
Allow the cake to cool slightly in the tin and dust with cocoa powder. Remove from the tin and allow to cool on a wire rack. Cut into pieces and serve the chocolate cake with whipped cream as desired.