Preheat the oven to 350°F (175°C). Grease and flour a 10-inch Bundt pan (or two 9×5-inch loaf pans).

Mix dry ingredients.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.

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

Add eggs and flavoring.
Beat in eggs one at a time, then add vanilla. Mix until smooth.

Incorporate sour cream and dry ingredients.
Alternate adding the flour mixture and sour cream to the batter, starting and ending with flour. Mix until just combined.

Make the pumpkin swirl.
In a separate bowl, combine pumpkin puree, brown sugar, and spices. Stir well. Fold in about 1½ cups of the plain batter into this pumpkin mixture to lighten it up.

Layer the batters.
Spoon half of the plain batter into the prepared pan, then add the pumpkin batter, and finally the remaining plain batter.
Use a knife or skewer to gently swirl the batters together for a marbled effect (don’t overmix).

Bake.
Bake for 60–70 minutes, or until a toothpick inserted in the center comes out clean.
(Loaf pans may take 50–55 minutes.)

Cool.
Let the cake cool in the pan for 15 minutes, then turn out onto a wire rack to cool completely.

Glaze (optional).
Mix powdered sugar, milk, and vanilla until smooth. Drizzle over cooled cake and let set before slicing.