Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
In a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt.
In another bowl, combine the buttermilk, vegetable oil, eggs, and 1 teaspoon of vanilla extract. Mix well.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Pour the batter into the prepared baking pan and smooth the top. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely in the pan.
While the cake is cooling, prepare the cream. In a small bowl, sprinkle the gelatin over the cold water and let it sit for about 5 minutes to bloom.
In a medium bowl, whip the heavy cream and powdered sugar until soft peaks form. Gently fold in the bloomed gelatin and 1 teaspoon of vanilla extract until well combined.
Once the cake has cooled, spread the whipped cream mixture evenly over the top. Refrigerate for at least 2 hours to set.
Cut into squares and serve chilled.