Preheat oven to 375 degrees F. Line two baking sheets with parchment paper.
In a medium bowl, whisk the flour, cocoa powder, baking soda, and salt until combined.
In the bowl of a stand mixer, beat the butter and brown sugar for 2 minutes using the paddle attachment until light. Add egg and vanilla, then beat again.
Alternate adding the flour mixture with the buttermilk, beating after each addition and scraping the sides of the bowl, just until combined.
Use a small scoop to drop batter onto the parchment-lined baking sheets, spacing them 2-3 inches apart.
Bake for 10 to 12 minutes, until just done around the edges and the tops bounce back when lightly touched. Cool on the baking sheet for 5 minutes before transferring the cookies to a wire cooling rack to cool completely.
Prepare the Frosting: Beat the butter in the bowl of an electric stand mixer for 3 to 4 minutes, until light and fluffy. Add powdered sugar, mixing on low speed until combined. Add the caramel sauce, vanilla, and salt, then beat again on medium-high speed for 2 minutes until light and fluffy. Transfer to a piping bag fitted with a round tip.
Assembly: Pipe a layer of frosting onto the flat side of one cookie, then top with another cookie to make a whoopie pie. Roll edges in additional sprinkles, if desired.
Store in an airtight container for up to 1 week.