Preheat Oven: Preheat to 350°F (175°C). Grease and flour a 9-inch round or 8×8-inch square baking pan.

Mix Dry Ingredients: In a medium bowl, whisk together flour, baking powder, baking soda, and salt.

Cream Butter and Sugar: In a large bowl, beat butter and granulated sugar until light and fluffy (about 3–4 minutes).

Add Eggs & Vanilla: Beat in eggs one at a time. Stir in vanilla extract.

Combine with Dry Ingredients: Add dry ingredients in thirds, alternating with sour cream, beginning and ending with the dry mixture. Mix until just combined.

Make Cinnamon Layer: In a small bowl, mix brown sugar, cinnamon, and walnuts (if using).

Layer the Batter: Pour half of the batter into prepared pan. Sprinkle half of the cinnamon mixture on top. Add the remaining batter and finish with the rest of the cinnamon mix.

Swirl: Use a knife or skewer to gently swirl the cinnamon layer into the batter.

Bake: Bake for 35–40 minutes or until a toothpick inserted in the center comes out clean.

Cool & Serve: Let cool in pan for 10 minutes, then transfer to a wire rack to cool completely. Slice and enjoy.