Preheat oven to 180Ā°C (160Ā°C fan). Grease and flour an 8- or 9-inch round cake pan.
In a heatproof bowl over a double boiler (or microwave in short bursts), melt the dark chocolate and butter together until smooth. Let cool slightly.
In a large bowl, beat the eggs and sugar until pale, thick, and fluffy.
Gently fold the melted chocolate mixture into the eggs and sugar.
Sift in the flour and fold it in just until combined. Avoid overmixing.
Pour the batter into the prepared pan and bake for 25ā30 minutes, or until a toothpick inserted comes out clean. Cool the cake in the pan for 10 minutes, then turn out onto a rack and cool completely.
2ļøā£ Prepare the Custard Cream
In a saucepan, whisk together the milk, flour, cornstarch, and sugar.
Cook over medium heat, stirring constantly, until thickened and smooth (about 5ā7 minutes).
Remove from heat and let it cool for a few minutes. Add the egg yolks and vanilla, and mix well while warm (but not hot).
Let the custard cool to room temperature. Once cooled, beat in the softened butter until light and creamy. Set aside.
