Preheat oven to 350°F. In a 9-inch metal cake pan over medium heat, melt ½ cup sugar with 3 tbsp butter, stirring until deep amber. Pour to coat the bottom, then arrange orange slices in a single layer.

In a bowl, beat the remaining 6 tbsp butter and ½ cup sugar until fluffy, about 3 minutes. Beat in eggs one at a time, then add the orange zest.

Whisk flour, baking powder, and salt together. Fold into the butter mixture in three additions, alternating with the orange juice in two additions, until just combined.

Spoon the batter over the oranges, spread it smooth, and bake for 40 to 45 minutes until golden and a toothpick comes out clean.

Cool the cake in the pan for 10 minutes. Run a knife around the edge, invert onto a plate, and serve warm or at room temperature.