Preheat oven to 350°F (175°C). Generously grease and flour 10-cup Bundt pan or 9x13-inch baking dish. For Bundt, get into all crevices—any ungreased spot sticks. Tap out excess flour.

In medium bowl, whisk together flour, baking powder, and salt until evenly combined. Set aside—add to wet ingredients later.

In large bowl using electric mixer, beat softened butter and sugar on medium-high speed 3-4 minutes until light, fluffy, and pale in color. Crucial step—proper creaming incorporates air making cake tender and creates perfect crumb. Don't rush. Should look almost white and significantly increased in volume.

Add eggs one at a time, beating well after each until fully incorporated before adding next. Prevents curdling. After all eggs in, beat in vanilla extract until combined.

With mixer on low, add flour mixture in three additions, alternating with buttermilk in two additions. Start and end with flour: add ⅓ flour mixture, mix briefly, add ½ buttermilk, mix briefly, add another ⅓ flour, mix, add remaining buttermilk, mix, then add final ⅓ flour. Mix each addition just until combined—should still see few streaks flour when add next ingredient. Once everything added, mix just until no dry flour remains. Do NOT overmix or cake will be tough.

Pour batter into prepared pan and spread evenly, smoothing top with spatula. Tap pan gently on counter few times to release air bubbles. Bake 45-55 minutes for Bundt (35-40 minutes for 9x13) until golden brown and toothpick inserted in center comes out clean or with just few moist crumbs—no wet batter.

Cool in pan 10-15 minutes. Brief rest lets firm up just enough to release from pan without falling apart. After 10-15 minutes, turn Bundt out onto wire rack placed over baking sheet (to catch drips later). If using 9x13, can leave in pan.

While cake cools slightly, make sauce. In small saucepan, combine butter, sugar, and buttermilk. Bring to boil over medium heat, stirring constantly to prevent scorching. Once boiling, let boil 1-2 minutes, still stirring, until slightly thickened and sugar completely dissolved. Remove from heat and stir in vanilla extract and pinch salt.

Using wooden skewer, fork, or handle of wooden spoon, poke holes all over top (or all surfaces if Bundt) of still-warm cake. Holes essential—let sauce soak deep into cake instead of just running off top. Slowly pour warm buttermilk sauce all over cake, letting sink into holes and soak in. Use all sauce. Might seem like too much but trust process.

Let cake sit at least 15-20 minutes to absorb sauce. Longer sits, more sauce penetrates and more amazing texture becomes. Serve warm or room temperature, slicing generous portions and spooning any extra sauce from bottom over each piece.