Preheat oven to 350°F. Pour the melted butter into a 9-inch round cake pan. Sprinkle the sugar over the butter, then layer the orange slices so that they overlap a little bit. Set the pan aside.
Whisk the flour, baking powder, baking soda, and salt together in a small bowl.
In a separate large bowl, use an electric mixer to beat the sugar and oil together. Add the eggs, orange juice, and zest, and mix until combined.
Mix in the sour cream.
Add the dry ingredients and mix until the batter becomes thick and dry. Then pour in the milk and mix until well combined.
Carefully pour the batter into the cake pan over the orange slices.
Bake for 45-50 minutes or until the top is golden brown and a toothpick inserted in the center of the cake comes out clean.
Let the cake cool in the pan for 30 minutes before inverting it onto a cake stand or platter to finish colling.
Slice and serve at room temperature.
Notes
Slice the oranges thin for best texture
Remove excess pith to reduce bitterness
Let the cake rest before flipping so topping stays intact
Use room temperature ingredients for even baking
Bake until the center is fully set to avoid sogginess