Start by preparing the custard; beat together the egg yolks and caster sugar in a mixing bowl until pale and thick. Combine the milk and vanilla extract in a small saucepan and bring to a simmer. Remove from the heat and pour over the egg and sugar mixture, whisking at simultaneously.
Pour the mixture back into the saucepan and cook over a low heat for 3-4 minutes until thickened, stirring all the time to make sure it doesn't catch on the bottom of the saucepan. Remove from the heat, transfer to a jug, cover and chill until needed. Pre-heat the oven to 200°C. Brush the inside of a silicone heart-shaped mould with the melted butter.
Dust the inside of the mould lightly with the cocoa powder, tipping out any excess. Chill until ready to fill with the batter. Prepare the batter by melting together the chocolate and cubed butter in a heatproof mixing bowl set over a pan of gently simmering water. Once melted, remove from the heat and allow to cool for 10 minutes.
Meanwhile, whisk together the eggs and egg yolks in a separate mixing bowl until light and thick. Sift the plain flour into the eggs and whisk until smooth. Pour the melted chocolate and butter mixture into the egg and flour mixture in thirds, mixing well between each addition.
Spoon the batter into the chilled mould.
Bake for 12-15 minutes until the cake looks light brown in colour on top, but slightly squidgy in the middle; a cake tester should come out a little unclean from the centre.
Remove from the oven and allow to cool for 2-3 minutes until the mould before turning out onto a serving plate.
Whisk the custard briefly until smooth then pour around the chocolate cake dessert. Serve immediately.
This dessert is very rich and indulgent, so it shouldn't be eaten too often. But it's perfect for those special occasions.