Classic Vanilla Butter Cake with Buttermilk Sauce

Preheat oven to 350°F (177°C). Lightly butter a $9\times13$-inch metal or ceramic baking dish, dust evenly 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 to 5 minutes until pale, fluffy, and noticeably increased in volume.

Add 4 room-temperature eggs one at a time while mixing on medium-low speed, allowing each to fully blend before adding the next. Stir in 2 tsp vanilla extract.

In a separate bowl, whisk 3 cups flour, 1 tbsp baking powder, and ½ tsp salt. With mixer on low speed, alternate adding flour mixture and 1 cup buttermilk in three stages (Flour $\rightarrow$ Buttermilk $\rightarrow$ Flour $\rightarrow$ Buttermilk $\rightarrow$ Flour) until just combined.

Spread batter into the prepared $9\times13$-inch pan. Bake at 350°F (177°C) for 35 to 40 minutes until top is golden-brown and a toothpick inserted in center comes out clean.

During the last 10 minutes of baking, combine ½ cup butter, 1 cup light brown sugar, and ½ cup buttermilk in a saucepan over medium heat. Bring to a gentle boil, stirring continuously, then simmer on low for 3 minutes. Remove from heat and stir in 1 tsp vanilla and ½ tsp baking soda (the sauce will foam up).

Remove hot cake from oven. Immediately poke holes 1 inch apart across the entire surface using a wooden skewer or fork. Slowly pour warm, foaming sauce over hot cake. Let sit undisturbed for 15 to 20 minutes to absorb sauce before serving warm.