Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan with non-stick spray or butter, then line the bottom with parchment paper.

In a small saucepan over medium heat, melt 1/4 cup of butter with 1/3 cup of brown sugar, stirring until dissolved. Once combined, pour this mixture into the bottom of the prepared pan.

Arrange the grapefruit, oranges, and lemon slices over the sugar-butter mixture, overlapping them to create a beautiful design.

In a large bowl, cream together 1/2 cup of softened butter and 1 cup of granulated sugar until light and fluffy (about 3-4 minutes).

Beat in the eggs one at a time, then add the vanilla extract and milk, mixing until smooth.

In a separate bowl, whisk together the flour, baking powder, and salt. Gradually incorporate this dry mixture into the wet mixture, mixing just until combined.

Carefully pour the cake batter over the arranged citrus slices in the pan, spreading it evenly.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for about 10 minutes, then run a knife around the edges, carefully invert it onto a serving plate.

Once flipped, remove the parchment paper and admire the beautiful ombré design before slicing up this delectable treat.