Preheat oven to 350°F

Lightly coat a springform pan, square pan, or round cake pan with cooking spray and line with parchment paper for easy removal

Combine all of the streusel ingredients in a small bowl using a pastry blender or fork until coarse crumbs form then set aside

In a medium bowl, whisk together the dry ingredients: flour, baking powder, baking soda, and salt then set aside

In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, cream butter and sugar until light and fluffy (about 2–3 minutes) on medium speed

Beat in eggs one at a time, then mix in vanilla extract, sour cream, and lemon juice until smooth

Reduce the speed to low and gradually add the dry ingredients to the wet ingredients, mixing until just combined

Gently fold in the juicy blackberries with a spatula. If your berries are especially large, consider halving them so they distribute more evenly

Spread the batter into the prepared pan

Sprinkle the crumb mixture on top

Bake for 45 minutes or until a cake tester inserted into the center of the cake comes out with just a few moist crumbs attached

Let cool in the pan for 10–15 minutes, then transfer to a wire rack to cool completely