• Preheat the oven to 350°F. Spray an 8x8 pan with cooking spray and line with parchment paper. Set aside.

Make the crumb topping.

• In a small, microwave safe bowl, melt the butter. Set aside.
  3 Tablespoons unsalted butter
• In a medium bowl, whisk together flour, both sugars, cinnamon, and salt.
  ½ cup plus 1 Tablespoon all-purpose flour, 2 Tablespoons light brown sugar, 1 Tablespoon granulated sugar, ½ teaspoon ground cinnamon, 1 pinch fine sea salt
• Add melted butter to the flour mixture and toss with a fork until fully moistened.

Make the cake batter.

• In a large bowl, whisk to combine flour, sugar, baking powder, baking soda, and salt.
  1 ¼ cup all-purpose flour, ¾ cup granulated sugar, ¾ teaspoon baking powder, ¼ teaspoon baking soda, 1 pinch fine sea salt
• Cut the butter into squares and add it to the bowl with the flour mixture. Using a pastry cutter, cut the butter into the flour mixture until it's the texture of coarse meal.
  ¼ cup unsalted butter
• In the same bowl you used to melt the butter for the crumb topping, whisk together the egg, sour cream, milk, and vanilla until well blended.
  1 large egg, ½ cup full-fat sour cream, 2 Tablespoons whole milk, 1 teaspoon vanilla extract
• Add the wet ingredients to the bowl with the flour and stir just until blended.

Assemble and bake the coffee cake.

• Add about two-thirds of the cake batter to the prepared pan. Use a rubber or offset spatula to spread the batter to the edges of the pan.
• Spoon the preserves over the batter. Drag a butter knife through the preserves and batter to marble the two together.
  2 Tablespoons blackberry preserves
• Add half of the blackberries over the batter.
  ¾ cup fresh blackberries
• Add the remaining cake batter over the berries, using the spatula to gently and carefully spread the batter to the edges of the pan.
• Spread the remaining blackberries over the top of the batter, followed by the crumb mixture. Use your fingers to crumble it into small crumbs and ensuring the batter is evenly covered and the crumbs reach to the edges.
• Bake the cake for about 35 minutes or until the cake is puffed and golden brown and the top springs back when gently poked. Let the cake cool in the pan on a wire rack for at least 30 minutes.

Make the glaze.

• In a small bowl, whisk together the confectioners' sugar and milk. The mixture should be thick but pourable. Spoon the glaze over the cake.
  ¼ cup confectioners' sugar, 1 ¼ teaspoons whole milk
• Remove the cake from the pan, slice, and serve. Be sure the cake is completely cool before storing.