Preheat the oven to 350°F (180°C). Lightly butter a 9-by 13-inch (23-by 33-cm) baking pan and line with a piece of parchment paper that hangs a little over the 2 long sides.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, and sugar.
Make a well in the middle, add the eggs, oil, buttermilk, and vanilla, and then whisk until the batter is smooth and combined, 1 to 2 minutes. Pour in the boiling water and whisk briefly to combine. The batter will be thin, but don’t worry, that’s what we want.
Pour the batter into the prepared pan. Bake until the cake is golden and a skewer inserted in the center comes out clean, 35 to 40 minutes.
☞TESTER TIP: Check the cake periodically and if it seems to be browning a little too much, tent with foil for the last 10 or so minutes of baking
Let the cake cool in the pan for 20 minutes before using the parchment paper handles to lift it onto a wire rack to cool completely.
Spread the buttercream over the cooled cake and top with sprinkles, if using. Cut into portions and serve. Store, covered, for up to 3 days.
*What can I do with that used vanilla pod?
Don’t discard the used pod. There’s a ton of flavor left inside. You can use it to roast fruits, to make vanilla sugar, to make your own vanilla syrup or to make your own vanilla extract with vodka or bourbon or rum…oh my!