4 tablespoons (1/2 stick) unsalted butter, at room temperature
For the Filling:
1/2 cup lightly packed light brown sugar
1 1/2 teaspoons ground cinnamon
1/4 teaspoon freshly grated nutmeg
1 1/2 teaspoons unsweetened cocoa powder
1/4 teaspoon kosher salt
For the Cake:
1/2 cup (1 stick) unsalted butter, at room temperature
1/2 cup granulated sugar
1/2 cup light packed light brown sugar
1 large egg
1 large egg yolk
1 teaspoon pure vanilla extract
2 cups all-purpose flour
1 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon kosher salt
1 cup buttermilk (or 3/4 cup milk or milk alternative + 1/4 cup yogurt or dairy-free yogurt alternative)
1 cups fresh blueberries
Place a rack in the upper third of the oven and preheat oven to 350 degrees F. Grease and flour a 9×9-inch square pan. For extra credit, line the pan with parchment paper and lightly grease the parchment, too.
To make the topping whisk together sugar, salt, flour and cinnamon. Add the butter and use a wooden spoon to break the butter into the dry ingredients until the mixture resembles a moist, coarse meal. Set aside.
To make the filling whisk together brown sugar, cinnamon, nutmeg, cocoa powder, and salt. Set aside.
To make the cake, in the bowl of a stand mixer fitted with a paddle attachment or in a large bowl with electric hand beaters, cream together butter and sugar until light and fluffy, about 4 minutes.
Add egg and yolk and beat on medium speed for 1 minute. Add vanilla and beat to combine. Stop the mixer and scrape down the sides of the bowl to ensure that everything is evenly combined.
In a medium bowl whisk together flour baking powder, baking soda, and salt. Add half of the dry ingredients into the butter mixture on low speed until just combined. Slowly stream in the buttermilk on low speed. Add the remaining dry ingredients and beat until just combined.
Add the blueberries and fold to evenly distribute.
Spread half of the batter into the prepared pan. Top with the filling. Dollop the remains ingredients on top of the filling and spread as evenly as possible from edge to edge. Use a butter knife to gently swirl the filling into the batter.
Sprinkle the topping evenly over the batter and bake for 45-55 minutes until a skewer inserted comes out clean or with a few moist crumbs. Remove from the oven and allow to rest for 15 minuets before slicing and enjoying. Cake will last, well wrapped. At room temperature for up to 5 days.