Position an oven rack in the lower third of the oven and preheat to 350 degrees F. Butter the bottom and sides of a 9-by-13-inch pan.
Whisk the flour, baking powder, baking soda and salt in a medium bowl; set aside. Whisk the buttermilk and vanilla in a spouted measuring cup; set aside.
Beat the granulated sugar and butter in a stand mixer (or a large bowl if using a hand mixer) at medium speed until very light in color and fluffy, about 5 minutes, scraping down the sides of the bowl as needed. With the mixer still running on medium, slowly add the eggs and beat until fully incorporated. Reduce the speed to the lowest setting; with the mixer running, add 1/3 of the flour mixture, then 1/2 of the buttermilk mixture, then 1/2 of the remaining flour mixture, then the remaining buttermilk mixture, then the remaining flour mixture. Scrape down the sides and beat until well mixed.
Spread the batter into the prepared pan. Bake until golden brown and a toothpick comes out clean when inserted in the center of the cake, about 30 minutes, rotating the pan halfway through. Let cool for 30 minutes in the pan. Use a large fork to poke holes an inch apart in the top and all the way through the cake.
Bring 1 cup water to a boil, pour over the gelatin in a medium bowl and stir until completely dissolved, about 2 minutes. Stir in 1/2 cup cold water. Pour the mixture evenly over the cooled cake. Refrigerate for 2 hours. Put a large bowl (for whipping the cream) in the refrigerator.
Remove the cake from the refrigerator. Whip the cream with the granulated sugar and vanilla in the chilled bowl until fluffy. Spread it over the cake and sprinkle the sanding sugar on top. Slice and serve.
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