Bakery-Style Blackberry Coffee Cake with Brown Butter Streusel

In a saucepan, combine blackberries, sugar, and lemon juice. Cook over medium heat until berries soften and release their juices.

In a small bowl, whisk cornstarch with water, then stir into the pan. Cook 1 to 2 minutes until thick and jammy.

When you drag a spoon through the mixture, it should leave a clear streak that slowly fills back in.

Set aside to cool (or refrigerate).

In a light-colored saucepan, melt butter over medium heat. Cook until foamy, golden, and nutty, about 5 to 7 minutes.

Pour into a heatproof bowl and cool until warm but not hot.

In a bowl, mix brown sugar, granulated sugar, flour, salt, and lemon zest.

Add the portion of browned butter and vanilla. Mix until crumbly.

Use your hands to form both large and small clumps.

Refrigerate while preparing the batter.

Preheat oven to 350°F (175°C). Line a 9-inch round pan with parchment.

Whisk browned butter with granulated and brown sugar.

Add eggs, yolk, vanilla, sour cream, and milk. Whisk until smooth.

In a separate bowl, whisk flour, baking powder, baking soda, and salt.

Fold dry ingredients into wet until just combined. Do not overmix.

Spread about 3/4 of the batter evenly in the pan.

Spoon blackberry filling over top and gently swirl.

Carefully spread remaining batter over the jam. Some mixing is fine.

Sprinkle streusel evenly over the top.

Bake 40 to 45 minutes, until a toothpick inserted in the center comes out with a few moist crumbs.

Cool in the pan for 30 minutes, then transfer to a wire rack to cool completely.

Dust with powdered sugar before serving.