1/2 cup (1 stick) Unsalted Butter, room temperature, plus more for pan
1/2 cup + 2 tbsp Granulated Sugar
1 tsp Orange Zest, finely grated
1 Large Egg
1 tsp Vanilla Extract
1 cup All-Purpose Flour
1/4 cup Cornmeal
1 1/2 tsp Baking Powder
1 tsp Ground Ginger
1/2 tsp Kosher or Sea Salt
1/2 cup Milk
Sour Cream Topping
1 cup Sour Cream
2 tbsp Granulated Sugar
Orange Zest (optional)
Preheat oven to 350°F with rack in center. Generously rub the bottom and sides of an 8-inch round cake pan with butter.
Pour blueberries into a strainer set over a saucepan and let drain; transfer drained berries to a bowl. Bring juice to a boil; whisk in 2 tablespoons sugar and reduce heat to a simmer. Simmer, turning down the heat as needed to prevent scorching, until juice has reduced to ¼ cup, 15-20 minutes. Pour juice into a measuring cup; if needed return juice to pan and continue cooking to reduce further, or add water until liquid measures ¼ cup. Stir in orange zest and reserved berries. Let cool.
In a large bowl or standing mixer, combine butter and remaining 1/2 cup sugar; beat until light and fluffy. Add egg and vanilla; beat until well combined. In another bowl, whisk together flour, cornmeal, baking powder, ginger, and salt. With mixer on low speed, add flour mixture to butter mixture in three batches, alternating with the milk, beginning and ending with flour mixture.
Pour cooled blueberries and syrup into the bottom of prepared cake pan. Using a spoon, dollop small amounts of batter over blueberries in pan; with the back of the spoon, gently smooth batter to edges of pan. (Don’t worry if some of the blueberry mixture gets mixed in to the batter as this is inevitable). Bake cake until a toothpick inserted in the center comes out clean, 30 to 35 minutes. Let cool on a wire rack for 20 minutes. Run a knife around edge of cake; invert onto a rimmed platter. Let cool completely. Slice cake and serve each piece with a dollop of sour cream topping. Garnish with slivers of orange zest, if desired.
Cover with a lid and refrigerate for 2 to 3 days. Use other berries in place of the blueberries (raspberries, blackberries, or boysenberries), and other nuts in place of the pecans (walnuts, pistachios, or almonds).