In a large bowl, whisk together flour, sugar, baking powder, salt, and cinnamon until evenly combined.

Cut the cold butter into the dry ingredients using your fingertips or a pastry cutter until the mixture resembles coarse crumbs.

Fold in the diced apples, coating them lightly in the flour mixture.

In a separate bowl, whisk eggs, milk, and vanilla until smooth and slightly frothy.

Pour the wet mixture into the dry ingredients and gently stir until a thick batter forms; avoid overmixing.

Transfer the batter to the prepared pan and spread it evenly with a spatula.

Bake for 45–55 minutes until the top is golden and a toothpick inserted in the center comes out clean.

Allow the cake to cool slightly in the pan before transferring to a wire rack.

For the custard, heat milk and cream in a saucepan over medium heat until steaming but not boiling.

In a bowl, whisk egg yolks and sugar until pale and slightly thickened.

Slowly pour the warm milk mixture into the yolks while whisking continuously.

Return the mixture to the saucepan and cook gently, stirring constantly, until it thickens enough to coat the back of a spoon.

Remove from heat, stir in vanilla, and strain for a silky texture.

Slice the cake and drizzle warm custard generously over each serving.