Oat Cake with Blueberries and Blackberries
This cake is best served warm but is also delicious later the same day.
- 1/2 stick unsalted butter, softened, plus more for pan
- 3/4 cup all-purpose flour, plus more for pan
- 1 cup plus 3 tablespoons quick-cooking oats, divided
- 2/3 cup water
- 1/2 teaspoon baking soda
- 1/2 teaspoon baking powder
- 3/4 teaspoon coarse salt
- 1/2 teaspoon ground cinnamon
- 1/2 cup granulated sugar
- 1/2 cup packed light-brown sugar
- 1 large egg
- 1 teaspoon pure vanilla extract
- 2 cups mixed ripe or overripe blueberries and blackberries, divided
- 3 tablespoons coarse sanding sugar
Preheat oven to 350 degrees. Butter and flour an 8-inch square metal baking pan, tapping out excess flour. Combine 1 cup oats and water and let stand until oats have softened, about 5 minutes. Meanwhile, whisk together flour, baking soda, baking powder, salt, and cinnamon in a separate bowl.
Beat together butter, granulated sugar, and brown sugar in a medium bowl with a mixer on medium-high speed until light and fluffy, about 3 minutes. Reduce speed to medium and beat in egg and vanilla until just combined, scraping down sides of bowl if necessary. Add flour and oat mixtures and beat on low speed until just incorporated. Fold in about half of berries.
Spread batter in pan and sprinkle with remaining 3 tablespoons oats and sanding sugar. Scatter remaining berries evenly over top. Bake until a toothpick inserted in center comes out with moist crumbs attached, 40 to 45 minutes. Let cool on a wire rack 30 minutes; cut into squares.