Preheat oven to 350°F. Lightly grease the inside of a 9-inch springform pan. Snugly line pan with a 12-inch circle of heavy-duty aluminum foil, pressing any pleats flat.

Bring brown sugar, honey, and 1/4 cup of the butter to a boil in a small saucepan over medium, stirring constantly. Remove from heat, and spread evenly in prepared pan.

Remove seeds from orange slices. Arrange in a single layer over sugar mixture.

Beat remaining 3/4 cup softened butter with a heavy-duty stand mixer at medium speed until creamy; gradually add granulated sugar, and beat until light and fluffy, about 3 minutes. Stir in zest and vanilla. Whisk together flour, cornmeal, baking powder, salt, and baking soda in a small bowl. Whisk together buttermilk and eggs in another small bowl. Add flour mixture to sugar mixture alternately with buttermilk mixture, beginning and ending with flour mixture. Beat just until blended after each addition. Spread batter evenly over oranges.

Bake in preheated oven until a wooden pick inserted in center comes out clean, about 1 hour, 10 minutes to 1 hour, 15 minutes, covering with foil after 1 hour if cake has browned on top but not cooked through completely. Cool in pan on a wire rack 15 minutes.

Transfer springform pan to a rimmed baking sheet (to catch any drips). Remove sides of pan and invert cake onto a serving platter. Cool 1 hour before serving.