Step 1

Preheat oven to 350°F and generously butter and flour a 10-cup bundt pan.

Step 2

Whisk together flour, baking powder, baking soda, and salt in a medium bowl.

Step 3

Beat butter and sugar in a large mixer bowl on medium speed until pale and fluffy. Reduce speed to low and add eggs one at a time; mix in vanilla. Beat in flour mixture in three additions, alternating with buttermilk (begin and end with flour mixture), just until incorporated.

Step 4

Spoon half the batter into prepared bundt pan and smooth with an offset spatula. Tap pan sharply to reduce air bubbles.

Step 5

Combine sugar, brown sugar, and cinnamon. Sprinkle evenly over the cake batter. Spoon remaining batter evenly over the cinnamon-sugar layer and carefully smooth with an offset spatula. Tap pan sharply again to reduce air bubbles.

Step 6

for 40 to 44 minutes, until cake is golden, springs back to the touch, and a tester inserted in the center comes out clean or with a few crumbs attached. Cool in pan on a wire rack for 10 minutes; carefully invert onto rack.

Step 7

Place rack with cake over a piece of wax paper. Combine sugar and cinnamon in a small bowl. Brush warm cake with melted butter and sprinkle generously with cinnamon-sugar. Let cake cool completely before serving.

Step 8

Store leftovers in an airtight container up to 2 days.