Preheat oven to 375°F. Line two baking sheets with parchment paper.
In a medium bowl, whisk the flour, cocoa powder, baking soda, and salt until combined.
2 cups all-purpose flour, 6 Tablespoons unsweetened cocoa powder, 1 1/4 teaspoons baking soda, 1/2 teaspoon salt
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.
1/2 cup salted butter, 1 cup light brown sugar, 1 large egg, 2 teaspoons vanilla extract
Alternate adding the flour mixture with the buttermilk, beating after each addition and scraping the sides of the bowl, just until combined.
1 cup buttermilk
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.
1 cup salted butter, 4 cups powdered sugar, 3/4 cup [caramel sauce], 1 teaspoon vanilla extract, 1/2 teaspoon coarse sea salt
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.