Topping: To peel the oranges, choose a very sharp knife and slice off the top and the bottom of the orange, just enough so you get a bit of the fruit. Set the orange flat on one of the cut surfaces. Now cut strips, top to bottom, deep enough to get the peel and the membrane, but as little of the fruit as possible. Continue cutting strips until the orange is peeled. Now, lay the orange on its side and cut through an orange section, just at the very edge against the membrane. Repeat on the other side of the section, then transfer the section to a strainer set over a bowl. Repeat for all the orange’s sections, then for the other oranges. (This process releases a lot of juice, so you’ll want to either hold the orange in a hand and work over a bowl, or work on a cutting board that has a groove around the edge to catch the juice.)

Melt butter over high heat, then whisk in sugar and orange peel. Spread in the bottom of a buttered 13"x9" pan, then arrange the orange slices on top of it.

Cake: Preheat oven to 350°. Whisk together flour, baking powder, and salt and set aside. Place the hazelnuts and the sugar in a food processor, pulse until the nuts are finely ground, then transfer to a mixing bowl. Add the butter and beat on medium speed until creamy, then add vanilla and eggs, one at a time. Turn the mixer speed to low and alternate adding a third of the flour then half the milk, repeating until it's all mixed together.

Spoon the batter atop the cake and bake until a tester comes out clean, about 55 minutes. Cool for 15 minutes, then invert onto a cooling rack.