Preheat oven to 350°F. Butter bottom and sides of 9x13 inch pan.
In a medium sized bowl, whisk together flour, baking powder, baking soda, and salt. Set aside. In a liquid measuring cup, whisk together buttermilk and vanilla. Set aside.
Using a mixer, beat sugar and butter in a large bowl on medium speed until light in color and fluffy, about 4 minutes. Scrape down sides of bowl. With mixer on medium speed, gradually add eggs until fully incorporated.
With mixer on lowest setting, gradually add dry and wet ingredients, alternating between the two until fully incorporated. Scrape down sides of bowl and beat until well mixed.
Spread batter evenly into prepared pan. Bake until golden brown and tester comes out clean when inserted to center, about 30 minutes. Rotate pan halfway through baking to ensure an even bake. Cool for 30 minutes. Using a fork, poke holes an inch apart in the top and all the way through cake.
Bring 1 cup of water to boil. Add gelatin to medium bowl and pour boiling water on top, stirring until dissolved. Stir in ½ cup cold water. Pour gelatin mixture evenly over cake. Refrigerate for 2 hours.
Whip heavy cream, vanilla, and sugar until fluffy and stiff peaks form. Spread whipped cream over cake and sprinkle sanding sugar on top. Chill until ready to serve.