(Makes 1 9-inch cake)
For the streusel:
1 stick unsalted butter, browned
1 cup packed dark brown sugar
1 cup all-purpose flour
1/2 tsp. ground cinnamon
1/2 tsp. kosher salt
To brown the butter:
Dice the butter and put it in a large saucepan over medium heat. Cook the butter, swirling it in the pan, until it turns a nutty brown. Be careful—it can go from browned to burnt very quickly. Immediately pour the butter into a shallow dish and refrigerate.
To make the streusel:
When the butter has chilled solid, dice it. Stir together the sugar, flour, cinnamon, and salt. Cut in cold browned butter with a pastry cutter until crumbly. Chill until ready to use.
For the cake:
2 cups kumquat slices, seeds removed (about 2 lbs. whole kumquats)
1¾ cups all-purpose flour
2 tsp. baking powder
1 tsp. baking soda
1/4 tsp. kosher salt
1 stick unsalted butter, browned, chilled until firm, and diced (follow the previous directions included for the streusel)
1 cup sour cream
1 cup sugar
1 tsp. pure vanilla extract
Preheat oven to 350°F.
Spray a nine-inch cake pan really well with nonstick spray and place a circle of parchment paper in the bottom. Place sliced kumquats in concentric circles on the bottom, covering as much as possible. Sprinkle the streusel mix over the kumquats.
Put the flour, baking powder, baking soda, and salt in a bowl and whisk together.
Put the cold brown butter, sour cream, sugar, and vanilla in the bowl of a stand mixer fitted with the paddle attachment and beat on medium speed until light and fluffy, about three minutes. Add the eggs one at a time, beating until well combined. Remove the bowl from the mixer, add dry ingredients all at once, and stir until just combined.
Pour the cake batter over the kumquats and streusel. Spread evenly. Bake until a skewer inserted into the center of the cake comes out clean, about 20 minutes. Let cool to room temperature and then flip out of the pan to reveal the kumquat top.