Preheat the oven to 350°F (176°C) and grease a rimmed baking sheet (also called a half-sheet pan) with butter. Line the bottom of the pan with parchment paper, and then grease the top of that with butter. Coat the pan with flour.
In a medium bowl, whisk the flour, baking powder and salt.
In a large bowl, cream the butter, sour cream and sugar until light and fluffy. Cream in the peanut butter. Then beat in the eggs one at a time, scraping down the sides of the bowl to be sure they’re thoroughly incorporated. Mix in the vanilla.
Gradually stir in the flour mixture on low speed, alternating with the buttermilk. Begin and end with the flour mixture.
Spread the batter evenly in the prepared pan and bake for 14-16 minutes, or until the edges are lightly browned and a toothpick inserted in the center comes out clean.
Let the cake rest in the pan for 5 minutes before turning it out onto a wire cooling rack lined with parchment or wax paper. Let the cake cool for a few minutes, and then place it in the freezer for 1 hour.