4. Make the Custard:
While the cake is baking, make the custard. In a saucepan, heat the double cream and whole milk over medium heat until just about to boil (don’t let it fully boil).
In a separate bowl, whisk together the egg yolks, sugar, vanilla extract, and a pinch of salt.
Slowly pour the hot cream mixture into the egg mixture while whisking constantly to temper the eggs.
Return the mixture to the saucepan and cook over low heat, stirring constantly until the custard thickens and coats the back of a spoon (about 5-7 minutes).
Remove from heat and let it cool slightly.
5. Make the Caramel Sauce:
In a medium saucepan, melt the sugar over medium heat, swirling the pan occasionally until the sugar has completely melted and turned a golden amber color.
Carefully add the butter to the melted sugar and stir until combined.
Slowly pour in the double cream and stir until smooth. Add a pinch of salt and remove from heat.
6. Assemble the Traybake:
Once the cake has cooled slightly, pour the custard over the top of the cake, spreading it evenly with a spatula.
Drizzle the warm caramel sauce over the custard layer.
Cut into squares and serve warm or chilled.