Prepare the Streusel

1. Combine Dry Ingredients: In a medium bowl, mix the brown sugar, cinnamon, chopped nuts (if using), and flour.
2. Add Butter: Stir in the melted butter until the mixture resembles coarse crumbs. Set aside.

Prepare the Cake Batter

3. Preheat Oven: Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking pan or bundt pan.
4. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
5. Cream Butter and Sugar: In a large bowl, use an electric mixer to beat the butter and sugar until light and fluffy, about 3 minutes.
6. Add Eggs and Vanilla: Beat in the eggs, one at a time, followed by the vanilla extract.
7. Alternate Flour and Sour Cream: Gradually add the dry ingredients to the wet mixture in three additions, alternating with the sour cream. Begin and end with the dry ingredients.

Assemble the Cake

8. Layer the Batter and Streusel: Spread half of the cake batter into the prepared pan. Sprinkle half of the streusel mixture evenly over the batter. Add the remaining batter on top and smooth with a spatula. Finish with the remaining streusel on top.
9. Bake: Bake for 50-55 minutes, or until a toothpick inserted into the center comes out clean.

Cool and Serve

10. Cool the Cake: Let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.
11. Optional Glaze: Drizzle with a simple glaze made from powdered sugar and milk for an extra touch of sweetness.