For the crumb topping:
1/2 cup all-purpose flour
1/3 cup firmly packed light brown sugar
1/4 tsp. kosher salt
1/4 cup unsalted butter, chilled

For the cake:
3 cups fresh blueberries, raspberries, or a mix
3 cups all-purpose flour more for the pan and for the berries
4 tsp. baking powder
1 tsp. baking soda
1/2 tsp. salt
3/4 cup unsalted butter, softened at room temperature; more for the pan
1-1/2 cups granulated sugar
3 large eggs
2 tsp. pure vanilla extract
1-1/2 cups buttermilk

Make the crumb topping:
Mix the flour, brown sugar, and salt in a medium bowl. Cut the butter into chunks and add to the dry mixture. Rub the flour and butter between your fingers until the mixture just comes together and has a nice crumbly texture.
Make the cake:
Rinse the berries well in a colander under running water and spread on paper towels. Let the berries air-dry for at least 15 min. Meanwhile, position a rack in the center of the oven and heat the oven to 350°F. Generously butter and flour a tube pan. In a medium bowl, mix the flour, baking powder, baking soda, and salt.
In the bowl of a stand mixer fitted with a paddle attachment or with a hand-held electric mixer, whip the butter and sugar together on medium-high speed until light and fluffy, about 4 min. Add the eggs one at a time, beating well for 15 seconds after each addition. Scrape the bowl, add the vanilla, and continue whipping until the mixture is light and fluffy, 2 min. On low speed, add the dry ingredients one-third at a time, alternating with the buttermilk 1/2 cup at a time. After the last addition of buttermilk, scrape the bowl, increase the speed to medium and beat for about 15 seconds to mix the batter fully.
Transfer the berries to a medium bowl and toss gently with 2 tsp. flour. Gently fold the berries into the cake batter with a rubber spatula to avoid crushing the berries too much. Scrape the batter into the prepared pan with the spatula, level the batter, and sprinkle with the crumb topping.
Bake until a toothpick or wooden skewer comes out clean when inserted into the middle of the cake, 55 min. Let the cake rest before turning it out onto a rack. Let cool completely and wrap tightly. Store at room temperature overnight before slicing.

To serve:
Slice the cake and serve each slice with a dollop of whipped cream or ice cream.
Make Ahead Tips
For the best flavor, make both the cake and its topping one day in advance