Preparation Steps

• Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
• In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, and salt.
• In a separate bowl, combine buttermilk, oil, egg, and vanilla extract. Add wet ingredients to dry ingredients and mix until just combined.
• Drop by rounded tablespoons onto prepared baking sheets. Bake for 8-10 minutes, or until set. Let cool on baking sheets for a few minutes before transferring to a wire rack to cool completely.
• For the filling: In a large bowl, beat butter until creamy. Gradually add powdered sugar, beating until smooth. Add milk and vanilla extract and beat until light and fluffy.
• Spread filling onto the flat side of half of the cookies. Top with remaining cookies, flat side down. Serve immediately or store in an airtight container.