Preheat your oven to 350°F (175°C). Grease a round cake pan with butter and line the bottom with parchment paper.

Thinly slice the oranges. Remove any seeds and set them aside.

In a small saucepan over medium heat, combine 1/2 cup of sugar and 1/4 cup of water. Stir until the sugar dissolves to create a simple syrup.

Once it starts to simmer, add the orange slices in a single layer and let them cook for about 5 minutes until they become tender.

Pour the syrup mixture into the prepared cake pan and arrange the cooked orange slices evenly over the bottom of the pan.

In a mixing bowl, cream the softened butter with the remaining 1/2 cup of sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Incorporate the vanilla extract.

In another bowl, combine the flour, baking powder, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, alternating with the milk. Mix until just combined, being careful not to overmix.

Pour the cake batter over the oranges in the pan, smoothing the top evenly.

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

Allow the cake to cool in the pan for 10 minutes before inverting it onto a serving plate.

Let it cool completely, then slice and enjoy!