In a small bowl, mix the brown sugar, cinnamon, and flour. Cut in the cold butter using a fork or pastry cutter until the mixture resembles coarse crumbs. Set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
In a large mixing bowl, use an electric mixer to cream the butter and sugar until light and fluffy, about 2-3 minutes.
Beat in the eggs one at a time, ensuring each is well incorporated. Stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Start and end with the dry ingredients, mixing just until combined.
Preheat the oven to 375°F (190°C). Grease and flour a 9-inch baking pan. Pour half the batter into the pan and spread evenly.
Sprinkle half the streusel mixture over the batter. Add the remaining batter and top with the rest of the streusel.
Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then transfer it to a cooling rack. Slice and serve warm or at room temperature.