Preheat the oven to 350°F (175°C).
Grease a 9-inch round cake pan.
In a large mixing bowl, cream together the softened butter and granulated sugar.
Add the eggs, one at a time, mixing well after each addition.
Stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with buttermilk.
Prepare the cinnamon swirl by mixing together brown sugar and cinnamon.
Pour half of the batter into the prepared cake pan, followed by the cinnamon swirl, then the remaining batter on top.
Top with chopped nuts, if desired.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack.
Serve warm or at room temperature, optionally drizzled with icing.
