Classic Vanilla Butter Cake with Buttermilk Sauce

Preheat oven to $350^\circ\text{F}$ ($177^\circ\text{C}$). Butter a $9\times13$-inch baking dish, dust with 1 tbsp flour, and tap out excess.

In a stand mixer fitted with the paddle attachment, combine 1 cup softened butter and 2 cups granulated sugar. Beat on medium-high speed for 4–5 minutes until light, pale, and fluffy.

Add 4 room-temperature eggs one at a time on medium-low speed, letting each incorporate fully. Stir in 2 tsp vanilla extract.

Whisk 3 cups flour, 1 tbsp baking powder, and $\frac{1}{2}$ tsp salt. With mixer on low, alternate adding flour mixture and 1 cup buttermilk in 3 stages until just combined.

Spread batter evenly into the prepared pan. Bake for 35 to 40 minutes until golden brown and a toothpick inserted in the center comes out clean.

During the last 10 minutes of baking, combine $\frac{1}{2}$ cup butter, 1 cup light brown sugar, and $\frac{1}{2}$ cup buttermilk in a saucepan over medium heat. Bring to a gentle boil, then simmer for 3 minutes. Remove from heat; stir in 1 tsp vanilla and $\frac{1}{2}$ tsp baking soda.

Remove hot cake from oven. Immediately poke holes 1 inch apart across the entire cake using a skewer. Pour warm foaming sauce over the hot cake. Rest undisturbed for 15–20 minutes before serving.