Step 1: Bake the Cake

1. Preheat the oven to 350°F (175°C). Lightly grease and flour an 8-inch round cake pan.
2. In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
3. In a larger mixing bowl, beat the softened butter and sugar until the mixture turns pale and fluffy.
4. Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract and lemon zest.
5. Alternately add the dry ingredients and the milk to the batter, mixing gently until just combined.
6. Pour the batter into the prepared pan, spreading it evenly.
7. Bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean.
8. Allow the cake to cool for 10 minutes in the pan, then transfer it to a wire rack to cool completely.

Step 2: Prepare the Lemon Custard

1. In a saucepan over medium heat, warm the milk and sugar until the sugar has dissolved.
2. In a small bowl, whisk together the egg yolks and cornstarch until smooth.
3. Slowly drizzle a bit of the hot milk mixture into the yolks, whisking constantly to temper them.
4. Return the yolk mixture to the saucepan and continue cooking over medium heat, stirring without stopping, until the custard thickens (about 5–7 minutes).
5. Remove from the heat and stir in the lemon juice, zest, and vanilla extract. Let the custard cool slightly before using.

Step 3: Assemble the Cake

1. Once the cake is completely cooled, slice it horizontally into two layers.
2. Spread half of the lemon custard over the bottom layer.
3. Place the second cake layer on top and spread the remaining custard across the surface.
4. Chill the assembled cake in the refrigerator for 1–2 hours before slicing and serving. This allows the flavors to settle and the custard to firm up beautifully.