Check off each step as you cook.

1. 01Make the custard first.Whisk custard sugar, cornstarch, yolks, and custard salt in a saucepan. Gradually whisk in custard milk. Cook over medium-low heat, stirring continuously, until thick bubbles appear; whisk for another minute at a gentle bubble. Check that it reaches at least 160°F / 71°C. Remove from the heat and stir in custard butter and vanilla.
2. 02Cool the filling.Pour custard into a shallow heatproof bowl. Set in an ice-water bath for about 20 minutes, stirring and keeping bath water out. Cover its surface and refrigerate for at least 2 hours until cold and thick.
3. 03Mix the cake batter.Heat the oven to 350°F / 180°C. Grease two 8-inch / 20 cm round cake pans and line their bases with parchment. Beat cake butter and cake sugar until pale and fluffy. Beat in whole eggs one at a time. Whisk flour, baking powder, and cake salt separately. Fold the dry mixture into the butter mixture in three additions, alternating with cake milk and vanilla, just until smooth.
4. 04Bake and cool the layers.Divide batter evenly between the pans. Bake on the same oven shelf for 25–30 minutes, until a skewer inserted near the center comes out clean and the tops spring back. Cool in the pans for 10 minutes, then turn out onto racks for about 40 minutes until completely cool.
5. 05Fill and glaze.Place one cold cake layer on a plate. Stir the cold custard smooth and spread over the cake, leaving a small border. Add the second cake gently. Heat heavy cream just until steaming, pour over chopped chocolate, and wait 2 minutes. Stir smooth, then leave for about 10 minutes until thick but pourable. Spread over the top, allowing a few natural drips.
6. 06Chill for neat slices.Refrigerate for at least 2 hours before cutting. Slice with a clean knife and keep any remaining cake refrigerated.

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