Grease a 9x13-inch pan with a 1-inch quarter sheet pan with cooking spray. Preheat the oven to 350F.

In a large bowl, whisk together the flour, sugar, salt, cinnamon and baking soda.

In a separate small saucepan or microwave-safe dish, combine the butter and peanut butter. Melt over low heat until smooth.

Next, add the buttermilk and egg to the melted butter mixture, and whisk until smooth.

Pour the wet ingredients over the dry ingredients, and stir to combine.

Pour into the prepared pan, and bake for 22-26 minutes. The cake will brown around the edges more than the middle. It will not pull away from the pan when done—use a toothpick to test for crumbs.

While the cake is still warm, make the frosting: add the butter, peanut butter, buttermilk, and powdered sugar in a small sauce pan. Heat over low until the frosting is melted and pour-able.

Stir in the vanilla, and then pour the warm frosting over the warm cake. Spread to the edges, and let set before slicing.