I preheat my oven to 350°F (175°C) and lightly grease a 9×9-inch baking pan. Sometimes, I also line it with parchment paper for easy cleanup.

In a small bowl, I whisk together flour, baking powder, baking soda, and salt. This helps the cake rise evenly and stay fluffy.

In a large bowl, I beat the butter and sugar until it’s light and creamy. It usually takes about 2–3 minutes. Then, I add in the eggs one at a time, beating well after each. I stir in the vanilla.

I alternate between adding the dry mixture and sour cream, starting and ending with the flour. I gently fold the batter until just combined—overmixing ruins the texture.

I gently fold in the berries with a spatula. If I’m using frozen berries, I toss them in a little flour to prevent them from sinking.

Prepare the Crumb Topping

In another bowl, I mix flour, brown sugar, and cinnamon. Then, I cut in the cold butter using a fork until it resembles coarse crumbs.

I pour the batter into the pan and spread it evenly. Then I sprinkle the crumb topping over the batter. I bake it for 38–42 minutes or until a toothpick comes out clean.

I let the cake cool in the pan for 10–15 minutes, then transfer it to a rack to finish cooling. It’s delicious warm or at room temperature.