Preheat & prepare – Preheat oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
Cream butter & sugar – In a large mixing bowl, beat the softened butter and granulated sugar until light and fluffy.
Add eggs & vanilla – Mix in the eggs one at a time, then stir in the vanilla extract.
Mix dry ingredients – In another bowl, whisk together the flour, baking powder, salt, and cinnamon.
Combine wet & dry – Gradually add the dry ingredients to the butter mixture, alternating with the milk, mixing until just combined.
Make cinnamon swirl – In a small bowl, mix the brown sugar and walnuts (if using).
Layer the cake – Spread half the batter into the prepared pan. Sprinkle half the cinnamon sugar mixture over the batter. Add the remaining batter, then top with the rest of the cinnamon sugar.
Create the swirl – Use a knife to gently swirl the topping into 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 cool for about 10 minutes before slicing. Serve warm or at room temperature.
