Step 3: Make the Buttermilk Sauce
9. While the cake is baking, prepare the sauce. In a medium saucepan over medium heat, combine the butter, sugar, buttermilk, and vanilla. Stir until the butter is melted and the sugar is dissolved.
10. Add the baking soda and whisk constantly. The mixture will foam up, so be sure to use a pan large enough to accommodate this. Remove from heat once the sauce thickens slightly (about 3–4 minutes).

Step 4: Assemble and Serve
11. Once the cake has cooled, poke holes all over its surface using a skewer or the handle of a wooden spoon.
12. Pour the warm buttermilk sauce evenly over the cake, allowing it to seep into the holes.
13. Let the cake rest for at least 30 minutes to absorb the sauce fully