Studded with bright, juicy blueberries, this spiced coffee cake has a hefty dose of cinnamon in the cake and the crumble, and a cinnamon ribbon running through the center. You can use blackberries or raspberries in place of the blueberries, or even chocolate chips for an extra-sweet treat. Serve warm slices of cake with a cup of coffee for the perfect weekend brunch or afternoon pick-me-up.
Heat oven to 350 degrees and set a rack in the center. Spray an 8-by-8-inch square pan with cooking spray and line the pan with a strip of parchment paper that hangs over two sides.
Make the crumb topping: In a medium bowl, combine the flour, light brown sugar, oats, cinnamon and salt. Add the butter and squish it together with your fingertips until it’s evenly moistened, and pebble-size crumbs form.
Make the cinnamon swirl: In a small bowl, combine the granulated sugar and ground cinnamon; set aside.
Make the cake: In a large bowl, beat the sugar and butter together with an electric mixer on medium-high speed until light and fluffy, about 4 minutes. Add the eggs one at a time, mixing in one egg completely before adding the next. Add the sour cream, cinnamon, vanilla and salt, and beat to combine.
Stop the mixer and add the 1 1/2 cups/195 grams flour, the baking powder and baking soda; mix on low speed until just combined. Toss the blueberries with the remaining 1 tablespoon flour, then gently fold them into the batter.
Assemble the cake: Spoon half the batter into the pan and smooth it to cover the bottom of the pan in an even layer. Sprinkle the cinnamon sugar evenly over the top of the batter. Spoon the remaining cake batter over the top and gently smooth it to cover the cinnamon sugar. Sprinkle the crumb topping over the top.
Bake the cake for 45 to 55 minutes until puffed and golden, and a tester inserted into the center comes out clean. Serve warm or at room temperature. Store the cake loosely wrapped at room temperature for up to 3 days.