Preheat the oven to 350°F (175°C) and grease a round cake pan.

Arrange orange and lemon slices in the bottom of the pan.

In a saucepan, melt 1/4 cup of butter with 1/2 cup of sugar until bubbly and golden; pour over the fruit.

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

In another bowl, cream the remaining butter and sugar until light; add the eggs one at a time along with the vanilla.

Gradually mix the dry ingredients into the wet mixture until just combined. Pour the batter over the fruit.

Bake for 30-35 minutes or until golden brown. Cool slightly before inverting onto a plate.