Preparation

Make the topping

• Butter a 9×2-inch round cake pan. Line the bottom with parchment and butter the parchment.
• Using a rasp grater, finely grate 2 tsp. zest from 1 orange and reserve for the cake. Halve the orange and squeeze to yield 3 Tbs. juice. Trim the ends off the other orange, quarter it lengthwise, and then slice the quarters crosswise 1/8 inch thick; set aside.
• Combine the butter and sugar in a 2-quart saucepan and cook over medium-low heat, stirring occasionally, until the mixture is a color slightly darker than peanut butter, about 5 minutes. Add the orange juice (be careful; it will bubble and steam). Cook, stirring, until combined, about 30 seconds. Immediately pour the caramel into the cake pan. Let sit until cool enough to touch, about 5 minutes.
• Decoratively arrange as many of the orange slices as fit comfortably on the caramel, leaving some space between for the almonds. Sprinkle the almonds between the orange slices.

Make the cake

• Position a rack in the center of the oven and heat the oven to 325°F.
• Spread the almonds on a rimmed baking sheet and toast, stirring once, until golden brown, about 6 minutes. Let cool. Pulse in a food processor until finely ground. Transfer to a medium bowl and combine with the flour, baking powder, and salt.
• In a stand mixer fitted with the paddle attachment (or in a large bowl using a handheld electric mixer), beat the butter and sugar on medium speed until light and fluffy, 3 to 5 minutes. Add the reserved orange zest and the almond extract, and mix until combined. Add the eggs, one at a time, beating well after each addition.
• With the mixer on low speed, alternately add the flour (in 3 additions) and the sour cream (in 2 additions), beginning and ending with the flour, and beating until just combined after each addition.
• With a spatula, gently scrape the batter into the prepared cake pan without disturbing the topping. Smooth the surface. Tap the pan firmly against the counter once or twice to remove any air bubbles. Bake, rotating halfway through, until a tester inserted in the center of the cake comes out clean, 40 to 45 minutes. Cool on a rack for 10 minutes. Run a knife around the edge of the cake to release it from the pan. Invert the cake onto a serving plate. Remove the pan, and rearrange any fruit or nuts that have stuck to the parchment. Serve warm or at room temperature.