Chocolate-Peppermint Whoopie Pies

Directions

1. PREHEAT oven to 350°F (180° C).
2. BLEND the butter, sugar, and lentils in the bowl of a food processor, scraping down the sides of the bowl, until well blended. Add the egg and vanilla and pulse until smooth.
3. STIR together the flour, cocoa, and salt in a small bowl. In a small dish, stir the baking soda into ¼ cup very hot water until dissolved.
4. ADD about a third of the dry ingredients to the food processor and pulse just until combined, then add half the buttermilk and half the baking soda mixture in the same manner. Repeat with another third of the dry ingredients, the rest of the buttermilk and baking soda mixture, and then the remaining dry ingredients, scraping down the side of the bowl and pulsing just until blended.
5. DROP large, round spoonfuls of batter onto a parchment-lined baking sheet, leaving at least an inch between them. Bake for 12-15 minutes, until the tops are springy to the touch. Transfer to a wire rack to cool.
6. TO make the frosting, in a mixing bowl, beat the butter, sugar, milk, mint extract, and salt until fluffy and smooth; add a little extra sugar or milk if needed to achieve a spreadable consistency. Once completely cooled, spread frosting on the bottoms of half the cookies, and sandwich with a second cookie. Repeat until all cookies and frosting are used up.