Prepare all ingredients at room temperature. Clean the lemon thoroughly to remove wax, then zest it and mix the zest with sugar. Line the baking tin with parchment paper.

Preheat the oven to 170°C (about 20 minutes).

Cream the butter with salt and the sugar mixed with lemon zest until fluffy.

Gradually add beaten eggs in three portions, whisking until the mixture doubles in volume.

Sift flour and baking powder twice, then fold them into the mixture using a spatula.

Add lemon juice, rum, and milk, and mix lightly until combined. Use a whisk to remove lumps if necessary.

Slice the lemon and coat it with sugar. Arrange the slices at the bottom of the lined tin.

Pour the batter over the lemon slices and bake at 170°C for 40-50 minutes.

Cool the cake upside down in the tin, ensuring the lemon slices face up.