Make the whoopies
Position an oven rack in the center of the oven and heat the oven to 350°F. Line three cookie sheets with parchment or nonstick liners.
Whisk the flour, cocoa powder, baking powder, baking soda, and salt in a medium bowl until well blended and no lumps remain. Put the butter and granulated sugar in a large bowl. Beat with an electric mixer fitted with the paddle attachment on medium speed until well blended and smooth, about 3 minutes. Add the eggs, one at a time, beating until just blended between additions. Add the vanilla and food coloring with the last egg. Stop to scrape down the bowl and the beater as needed. Add half of the flour mixture and mix on low speed until just blended. Add the sour cream and mix just until blended. Add the remaining flour mixture and mix on low speed until just blended.
Using a 1 Tbs. mini scoop, shape the dough into balls and arrange about 1-1/2-inches apart on the prepared cookie sheets. Bake, one sheet at a time, until a toothpick inserted in the center of one whoopie comes out clean, 10 to 12 minutes. Move the sheet to a wire rack, let the whoopies sit for 10 minutes, and then transfer them to a wire rack to cool completely.
Make the filling
Put the butter and Fluff in a large bowl. Beat with an electric mixer fitted with the whisk attachment on medium speed until well blended and smooth, about 2 minutes. Add the confectioners’ sugar, vanilla, and salt. Continue beating until smooth and fluffy, about 2 minutes.
Assemble the whoopie pies
Turn over half of the cooled whoopies so they are flat side up. Spoon a slightly rounded 1 tablespoon of the filling onto the center of each whoopie. Top with the remaining whoopies, flat side down. Press gently on each top to spread the filling almost to the edge. Roll the edges in the sprinkles, if using, to coat the frosting. Serve immediately or chill for 30 minutes. Whoopie pies are best when served at room temperature.
Make Ahead Tips
The whoopie cakes can be baked, covered, and stowed at room temperature for up to 1 day before filling.
Filled whoopies can be stowed in the refrigerator for up to 2 days. Serve at room temperature.
Photo: Miki Duisterhof