Sift the flour, baking powder and cocoa powder into a bowl. Add the butter, sugar and eggs and whisk until the mixture is smooth.
Grease a 23 cm (9 inch) springform or cake tin and line the base. Turn the mixture into the tin and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20–25 minutes until just firm. Cool on a wire rack. Transfer to a serving plate and drizzle with the liqueur or juice.
Cut a strip of greaseproof paper 1 cm (½ inch) longer than the sponge circumference and 6 cm (2½ inches) deep. Melt the chocolate (see
Sprinkle the gelatine over the measured water and leave to soften. Beat the egg yolks in a bowl with the sugar, cornflour and a little milk. Bring the remaining milk to the boil. Pour it over the egg yolk mix, whisking. Return to the pan and cook gently until thickened.
Remove from the heat and stir in the gelatine until dissolved. Add 200 g (7 oz) chocolate and leave until melted. Stir smooth, then leave to cool until thickening.
Whip the cream to peaks and fold into the chocolate mix. Turn into the case on top of the sponge and level. Chill for 1–2 hours until set. Remove the paper strip. Lay the chocolate shards on top to create a nest. Pile the mini eggs in the centre. Chill until ready to serve.