Preheat Oven & Prepare Pan
Preheat oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan or a bundt pan.
Make the Streusel
In a small bowl, combine brown sugar, cinnamon, and nuts (if using). Set aside.
Mix Dry Ingredients
In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
Cream Butter & Sugar
In a large mixing bowl, beat the butter and granulated sugar together until light and fluffy, about 3 minutes.
Add Eggs & Vanilla
Beat in eggs one at a time, then add vanilla extract.
Incorporate Dry Ingredients
Add the dry mixture in three parts, alternating with sour cream and milk. Begin and end with dry ingredients. Mix until just combined — do not overmix.
Assemble the Cake
Spread half the batter in the prepared pan, sprinkle with half of the streusel mixture. Spread the remaining batter over the top, then sprinkle the remaining streusel evenly.
Bake
Bake for 40–45 minutes (rectangular pan) or 50–55 minutes (bundt pan), or until a toothpick inserted in the center comes out clean.
Cool & Serve
Let cool for at least 15–20 minutes before slicing. Serve warm or at room temperature.