Preheat the oven to 180°C (350°F). Line a 12-hole muffin tin with cupcake liners.

Warm the milk in a small saucepan over medium-low heat until just steaming. In a bowl, whisk the egg yolks, sugar, and cornstarch until smooth. Slowly whisk in the warm milk, then return the mixture to the saucepan. Cook over low heat, whisking constantly, until thickened. Stir in the vanilla, transfer to a bowl, cover the surface directly, and let cool.

Stir together the flour, brown sugar, and cinnamon in a small bowl. Add the melted butter and mix with a fork until chunky crumbs form. Set aside.

In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt.

In a large bowl, beat the softened butter and sugar until light and fluffy. Beat in the eggs one at a time, then mix in the vanilla.

Add the dry ingredients in two additions, alternating with the milk, and mix just until combined. Fold in the diced apple gently.

Spoon batter into the cupcake liners, filling each about halfway. Add a small teaspoon of cooled custard to the center of each, then top with a little more batter so the custard is mostly covered.

Scatter the crumble topping generously over each cupcake, pressing very lightly so it adheres.

Bake for 20 to 22 minutes, until the tops are golden and a toothpick inserted into the cake portion comes out with a few crumbs but no wet batter.

Cool the cupcakes in the pan for 5 minutes, then transfer to a wire rack to cool completely before storing or serving.