Preheat oven to 350°F or 175°C and grease a 9x13 inch baking pan thoroughly.
Whisk flour, granulated sugar, cinnamon, baking powder, baking soda, and salt until evenly combined.
Add eggs, buttermilk, oil, and vanilla, whisking until the batter is smooth.
Pour half of the batter into the prepared pan and spread evenly.
Mix brown sugar and cinnamon, then sprinkle evenly over the batter.
Pour remaining batter on top and gently swirl with a knife.
Bake for 35–40 minutes until lightly golden and a toothpick comes out clean.
Whisk glaze ingredients, drizzle over warm cake, then slice and serve.