Step 2: Mix the Batter
In a large mixing bowl, beat the softened butter with an electric hand mixer or whisk until smooth and creamy — about 30 to 60 seconds. Add the sugar and continue beating until the mixture is light, pale, and noticeably increased in volume, about 3 to 4 minutes. This creaming step is important: the friction between the butter and sugar creates tiny air bubbles that give the finished cake its characteristic light, tender crumb. Do not rush it. Add the eggs one at a time, beating well after each addition and scraping down the sides of the bowl to ensure everything is evenly incorporated. Add the vanilla extract and mix briefly to combine.
In a separate bowl, whisk together the flour, baking powder, and salt. Add the flour mixture to the butter mixture in three additions, alternating with the milk in two additions — beginning and ending with the flour. This means you add one-third of the flour mixture, mix until just combined, then half the milk, mix, then another third of the flour, then the remaining milk, then the final third of the flour. Mix only until the last streaks of flour disappear — do not beat the batter aggressively at this stage, as overworking develops gluten and produces a tough, dense cake rather than the tender crumb you want. The batter should be smooth and pourable, similar in consistency to thick pancake batter.
Step 3: Bake
Pour the batter into the prepared pan and spread it evenly to the edges with a spatula. Tap the pan gently on the counter two or three times to release any large air bubbles. Bake for 30 to 35 minutes, until the top is golden and a toothpick or thin skewer inserted into the center of the cake comes out clean or with just a few moist crumbs attached. The edges of the cake should be pulling away slightly from the sides of the pan, and the top should spring back when pressed lightly in the center. Do not open the oven during the first 25 minutes of baking, as the sudden change in temperature can cause the cake to sink in the middle.
Step 4: Make the Buttermilk Sauce
About 10 minutes before the cake is done, begin preparing the sauce so it is ready to pour while the cake is still warm. Combine the butter, sugar, buttermilk, and vanilla extract in a medium saucepan over medium heat. Stir continuously until the butter has melted and the sugar has dissolved — about 3 to 4 minutes. Remove the pan from the heat and stir in the baking soda. The mixture will foam up briefly as the baking soda reacts with the buttermilk — this is normal and expected. The sauce should be thin and pourable, with a warm amber color and a smell that is simultaneously sweet, buttery, and slightly tangy.
