Preheat oven to 350°F (175°C). Grease an 8x8 inch baking pan with butter or nonstick spray.

Make the streusel: In a small bowl, combine brown sugar, 1/4 cup flour, and cinnamon. Cut in cold butter using a pastry blender or your fingertips until mixture resembles coarse crumbs. Stir in nuts if using. Set aside.

In a medium bowl, whisk together 1 1/2 cups flour, baking powder, baking soda, and salt.

In a large bowl, beat softened butter and granulated sugar with an electric mixer on medium speed until light and fluffy, about 2 minutes. Scrape down sides.

Beat in egg and vanilla until well combined.

With mixer on low speed, add the flour mixture in three additions, alternating with buttermilk, beginning and ending with flour. Mix just until combined—do not overmix.

Spread batter evenly into prepared pan. Sprinkle streusel evenly over the top.

Bake for 28-32 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs. The top should be golden and the edges pulling away from the pan.

Cool in the pan on a wire rack for 15 minutes, then serve warm or at room temperature.