Make the streusel: combine flour, brown sugar, and cinnamon. Work in cubed butter with fingers until clumpy. Chill.

Preheat oven to 350°F (175°C). Butter a 9-inch springform or round cake pan.

Beat softened butter and granulated sugar until pale and fluffy, about 3 minutes. Beat in eggs one at a time, then vanilla.

Whisk the 2 cups flour, baking powder, and salt. Add to butter mixture in three additions, alternating with buttermilk in two additions, mixing just until combined.

Fold in blueberries gently with a spatula. Scrape batter into prepared pan and smooth the top.

Sprinkle chilled streusel evenly over the batter. Bake 45-55 minutes until golden and a toothpick in the center comes out with moist crumbs (tent with foil if browning too fast).

Cool in the pan on a wire rack for 20 minutes, then remove sides or invert. Slice and serve warm or at room temperature.