Mint Chocolate Whoopie Pies

For whoopie pies:

• 1 2/3 cups (212 grams) all-purpose flour
• 2/3 cup (57 grams) Dutch-process cocoa powder
• 1 1/2 teaspoons baking soda
• 1/2 teaspoon fine sea salt
• 1 stick (113 grams) unsalted butter, at room temperature
• 1 cup (200 grams) lightly packed dark brown sugar
• 1 large egg, at room temperature
• 1 teaspoon vanilla extract
• 1 1/4 cups (296 grams) buttermilk, at room temperature

For mint filling:

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