How to Make It
Make cookies: Preheat oven to 375ºF. Line 2 large baking sheets with parchment. In a bowl, whisk sour cream, egg, vanilla and vinegar.
In a large bowl, whisk flour, brown sugar, cocoa powder, baking soda and salt, breaking up lumps of sugar. Add butter and beat with an electric mixer on low until butter pieces are no larger than small peas. Stir in sour cream mixture and mix on low speed until moistened. Add food coloring; beat on low until incorporated, then beat on medium-high until well combined, scraping down sides of bowl periodically.
Using a small ice cream scoop, drop batter onto baking sheets, 2 1/2 inches apart. Bake cookies, 1 sheet at a time, until tops are cracked, 8 to 9 minutes. Remove from oven; let stand for 5 minutes. Transfer cookies to a wire rack to cool. Repeat with remaining batter.
Make filling: In a bowl, beat butter and sugar with an electric mixer on low until combined. Beat in corn syrup, vanilla and salt. Stir in Fluff and food coloring. Beat on high speed until light, about 2 minutes. Refrigerate for 20 minutes.
Spread a rounded teaspoon of filling onto flat side of a cookie and sandwich with another cookie. Repeat with remaining cookies and filling.