Melt chocolate. Melt butter. Blend the two together. Beat eggs (room temperature) and fold into cooled mixture of chocolate and butter. Add flour and sugar; mix. Pour into spring form pan and bake in pre-heated 400°F oven for 15 min. Cool immediately and leave in fridge overnight. Serve the small, very rich slices with either sour cream, whipped cream or, my favourite, a serving of thawed and mashed raspberries (unsweetened).