Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large mixing bowl, combine the flour, granulated sugar, baking powder, baking soda, and salt. Mix well.
In another bowl, whisk together the buttermilk, melted butter, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix; a few lumps are okay.
Spread the batter evenly into the prepared baking dish.
In a separate bowl, mix the brown sugar and water until well combined. Pour this mixture evenly over the batter in the baking dish. Do not stir.
Bake for 30-35 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Let the cake cool for about 10 minutes before serving. The cake will be gooey and delicious!