Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan or a 9x9 inch square baking dish.
In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Beat in the eggs one at a time, then mix in the sour cream and vanilla extract until well combined.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add the dry mixture to the wet ingredients, stirring until just combined.
In a small bowl, mix together the brown sugar, ground cinnamon, and chopped nuts (if using).
Pour half of the cake batter into the prepared pan and spread evenly. Sprinkle half of the cinnamon swirl mixture over the batter. Pour in the remaining batter and top with the remaining cinnamon swirl mixture, gently swirling it into the batter with a knife.
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan for about 10 minutes before transferring to a wire rack to cool completely. Dust with powdered sugar before serving if desired.
Notes
Dust with powdered sugar before serving if desired.