In a bowl, combine the flour, 1°C sugar, baking powder, 1 1/2 teaspoons 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.