Blood Orange Buttermilk Upside-Down Cake

Nonstick vegetable oil spray

• ⅓ cup demerara or raw sugar
• 2 blood oranges, very thinly sliced, seeds removed
• 2½ cups all-purpose flour
• ¾ cup plus 2 tablespoons granulated sugar
• 1 teaspoon baking powder
• 1 teaspoon baking soda
• ¾ teaspoon kosher salt
• 2 large eggs
• 1 large egg yolk
• 1½ cups buttermilk
• ¾ cup (1½ sticks) unsalted butter, melted, slightly cooled
• 1 teaspoon finely grated blood orange zest
• 2 tablespoons fresh blood orange juice

1. Preheat oven to 350°. Coat a 10" cake pan with nonstick spray. Scatter demerara sugar evenly in pan. Place orange slices on top (some overlap is fine).
2. Whisk flour, granulated sugar, baking powder, baking soda, and salt in a large bowl. Add eggs, egg yolk, buttermilk, butter, and blood orange zest and juice; mix until no dry spots remain (a few lumps are okay; do not overmix). Pour batter over oranges in pan and bake until top is golden brown and cake pulls away from sides of pan, 35–40 minutes. Transfer to a wire rack and let cool 5 minutes before inverting and turning out onto rack or a large plate.
3. Do Ahead: Cake can be baked 1 day ahead. Store tightly wrapped at room temperature.