In a bowl, combine the flour, 1 cup sugar, baking powder, 1½ tsp cinnamon, and salt.
In another bowl, combine the eggs, buttermilk, oil, and vanilla; stir into dry ingredients just until smooth.
Pour half the batter into a greased 9x5x3-inch loaf pan.
In a small bowl, combine the remaining sugar and cinnamon; stir in butter.
Drizzle half over batter; cut through with a knife to swirl.
Top with remaining batter. Drizzle with remaining cinnamon sugar mixture; swirl with a knife.
Bake at 350° for 55-60 minutes, or until a toothpick inserted near the center comes out clean.
Cool for 10 minutes before removing from pan to a wire rack.