Preheat the oven to 350°F (175°C) and prepare the baking pan.
In a large mixing bowl, cream together the softened butter and sugar until light and fluffy.
Add the eggs, one at a time, mixing well after each addition, then stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry mixture to the wet mixture, alternating with milk, starting and ending with the flour mixture.
Fold in the sliced strawberries gently to preserve their shape and freshness.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
While the cake cools, prepare the custard by combining milk, cornstarch, sugar, and lemon juice in a saucepan.
Cook the mixture over medium heat, stirring until it thickens and bubbles.
Once thickened, remove the custard from heat and stir in butter until melted and smooth.
Let the cake cool completely before spreading the custard on top.
Garnish with additional sliced strawberries on top of the custard for a fresh and appealing look.
Chill the completed cake in the refrigerator for at least 1 hour before serving.