Preheat oven to 350°F (175°C). Grease and flour the Bundt pan thoroughly.
In a bowl, whisk together flour, baking powder, and salt. Set aside.
Beat softened butter and granulated sugar until light and fluffy (3–4 minutes).
Mix in eggs one at a time, then add sour cream and vanilla extract.
Alternate adding flour mixture and milk, starting and ending with flour. Mix until just combined.
In a small bowl, mix brown sugar and cinnamon. Pour half of the batter into the Bundt pan, sprinkle cinnamon-sugar mixture, then top with remaining batter.
Use a butter knife to gently swirl in a figure-eight motion.
Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean.
Let cake cool in the pan for 15 minutes, then invert onto a wire rack. Cool completely.
Mix powdered sugar, milk, and vanilla extract into a smooth glaze. Drizzle over cooled cake before serving.
