Preheat the oven to 350° F.

Prepare the cake batter. In a large mixing bowl, whisk flour, sugar, baking powder, baking soda, and salt. In a separate bowl, beat sour cream, softened butter, eggs, and vanilla extract until well combined. Gradually add the dry ingredients to the wet, mixing until just combined, being careful not to overmix.

Make the streusel topping. In a small bowl, mix brown sugar, flour, melted butter, cinnamon, and chopped nuts.

Coat a 12 inch cast iron skillet with a nonstick baking spray like Baker’s Joy. Pour half of the cake batter into the skillet, spreading it evenly. Sprinkle ⅔ of the streusel topping over the batter, then add the remaining batter and top with the last of the streusel.

Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean. Let the cake cool.

Mix confectioner’s sugar and milk until dissolved. Drizzle over cooled cake, cut and serve.