Preheat & Prepare: Preheat oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan.

Cream Butter & Sugar: In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add Eggs & Vanilla: Mix in the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

Combine Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, salt, and ground cinnamon.

Mix Batter: Gradually add the dry ingredients to the wet mixture, alternating with the milk. Stir until just combined.

Prepare Cinnamon Swirl: In a small bowl, mix together the brown sugar and chopped walnuts (if using).

Assemble Cake: Pour half of the batter into the prepared baking pan and spread evenly. Sprinkle half of the cinnamon sugar mixture over the batter. Repeat with the remaining batter and cinnamon sugar.

Create Swirl Effect: Use a knife to gently swirl the batter for a marbled effect.

Bake: Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.

Cool & Serve: Let the cake cool in the pan for 10 minutes before slicing and serving.