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

Prepare the oranges by peeling them and removing the white pith. Slice them into thin rounds, making sure to remove any seeds.

In a small saucepan over medium heat, melt 1/4 cup of the sugar with 1 tablespoon of butter. Stir until the mixture becomes syrupy, about 5 minutes.

Pour this mixture into the prepared cake pan, evenly coating the bottom.

Arrange the orange slices in a single layer on top of the syrup. Set the pan aside.

In a mixing bowl, cream the remaining butter and sugar together until light and fluffy.

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

In another bowl, whisk together the all-purpose flour, almond flour, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the butter mixture, alternating with the sour cream and vanilla extract. Mix until just combined, then fold in the orange zest.

Pour the batter over the arranged oranges in the pan, smoothing it out gently.

Bake in the preheated oven for about 35-40 minutes or until a toothpick inserted into the center comes out clean.

Once baked, let the cake cool in the pan for about 10 minutes. Invert the cake onto a serving plate, removing the parchment paper carefully. Serve warm or at room temperature.