for the cinnamon swirl
6 tablespoons brown sugar
3/4 cup all-purpose flour
2 1/4 teaspoons ground cinnamon
pinch kosher salt
4 tablespoons unsalted butter, cold and cut into cubes
In a small bowl, combine brown sugar, flour, ground cinnamon, and salt. Use a fork to mix. Toss in butter and use your fingertips to work the mixture until it has the texture of rough cornmeal. Refrigerate until ready to use.
for the crumb topping
1/2 cup granulated sugar
1/3 cup cake flour
4 tablespoons unsalted butter, cold and cut into cubes
In a small bowl, combine sugar and cake flour. Use the tips of your fingers to work the butter into the flour mixture until the butter is in pea-sized pieces. Refrigerate until ready to use.
for the cake batter
3/4 cup unsalted butter, softened
1 1/2 cups granulated sugar
4 eggs
1 1/2 teaspoon vanilla extract
1 1/4 cups full fat sour cream
2 cups cake flour
1/2 cup all-purpose flour
2 teaspoons baking powder
1/2 teaspoon baking soda
1/4 teaspoon kosher salt
1/4 teaspoon freshly ground nutmeg
1/2 cup seedless blackberry preserves
1/2 cup fresh blackberries tossed with 1 tablespoon all-purpose flour (shake off excess flour)
Cook's Note: Tossing the berries in a bit of flour before adding them to the batter will prevent them from sinking to the bottom of the pan when baked.
Preheat oven to 350 degrees. Coat a 11.5x7.5 baking pan with nonstick spray.
Fit your stand mixer with the paddle attachment and beat butter on medium speed for one minute. Beat in sugar.
With the mixer on low speed, add eggs one at a time, scraping down the sides as needed. Add vanilla and sour cream.
Sift to combine flours, baking powder, baking soda, salt and nutmeg. With the mixer on low speed, slowly add the flour mixture until the batter is smooth.
Fold 2/3 of the batter into the prepared pan and top with preserves. Use a large spoon to place tablespoons of preserves over the batter, then use a butter knife to swirl the preserves into the cake batter. Press the flour-coated berries down into the batter and top with the cinnamon swirl mixture.
Fold the remaining batter into the pan and and smooth over with a spatula. Sprinkle with the crumb topping.
Bake for 50-60 minutes, until the cake is golden and a fork inserted into the center comes out clean. Allow to cool completely before serving (about 90 minutes). Enjoy!