1. Prepare the Custard Topping:

In a medium saucepan, combine egg, sugar, and cornstarch.

Stir in milk and cook over medium-low heat, whisking constantly until frothy and bubbling.

Continue to whisk and cook for 5–10 minutes until slightly thickened.

Remove from heat, stir in vanilla extract, and set aside to cool.

2. Prepare the Cake Batter:

Preheat oven to 350°F (175°C). Line a 9-inch tart or pie pan with parchment paper.

Peel, core, and dice apples. Toss with lemon juice in a bowl.

In a large mixing bowl, combine sugar, salt, and eggs. Beat with an electric mixer until pale and frothy.

Add vanilla extract and milk, mixing until combined.

Sift flour and baking powder into the bowl and blend until just mixed.

Fold in the apple pieces.

3. Assemble the Cake:

Pour the batter into the prepared pan and spread evenly.

Transfer custard topping into a Ziploc or piping bag, snip the tip, and pipe a swirl pattern on top of the batter, starting from the center outward.

4. Bake:

Bake for 30–40 minutes or until golden brown and set.

If top browns too quickly, cover loosely with foil during the last 10–15 minutes.

Let cool for at least 1 hour at room temperature before removing from the pan.

5. Serve:

To release the cake, gently invert using a flat plate, remove parchment, then flip it right-side up.

Dust with powdered sugar before slicing into 8–12 pieces.