Chocolate Mint Sandwich Cookies

Ingredients:

Cookie

• 110 g unsalted butter, softened
• 0.5 cup granulated sugar
• 1/3 cup light brown sugar
• 1/4 teaspoon coarse salt
• 2 large eggs + 1 large egg yolk
• 0.5 tsp vanilla extract
• 2 cups premium flour
• 0.5 cup cocoa powder + extra for sprinkling

Butter cream

• 220 g unsalted butter, cut into pieces
• 1/4 cup fresh mint leaves (about 20)
• 2 cups powdered sugar
• 2 tbsp. heavy cream
• A pinch of coarse salt
• A few drops of green food coloring
• 340 g semi-sweet chocolate, finely chopped
• 1 tbsp. vegetable oil
• Special equipment: pastry bag with a round tip

Preparation:

1. Preheat oven to 175°C. Line 2 baking sheets with parchment paper.
2. Cookie:
   
   In the bowl of a stand mixer, beat the butter with the white and brown sugars and salt on medium speed until light and fluffy, about 2 minutes. Add the eggs one at a time, beating well after each addition. Add the yolk and vanilla and beat until smooth. Reduce the mixer speed to low. Combine the flour and cocoa powder in a small bowl, then add them to the butter mixture in two batches, mixing well.
3. Place the dough on a piece of plastic wrap (the dough will be slightly wet, but it will set in the refrigerator). Form the dough into a disk and wrap tightly in plastic wrap. Refrigerate until completely set, about 1 hour.
4. Place a small amount of cocoa powder in a small bowl. Scoop 2 tablespoons of dough and roll into a ball. Dip the bottom of a jar or flat-bottomed glass into the cocoa powder and press the dough into a flat disk about 3 inches (7.5 cm) in diameter. Place on the prepared baking sheet and repeat with the remaining dough. Bake the cookies until set and starting to brown on the bottom, 9–11 minutes. Remove from the oven and let cool on the baking sheet for 2 minutes, then transfer to a wire rack and let cool completely, about 1 hour.
5. Meanwhile, prepare the butter cream.:
   
   In a small saucepan, melt the butter with the mint over medium-low heat. Reduce the heat to low and continue melting the butter, gently mashing the mint, until the butter begins to infuse with the minty aroma, about 15 minutes. Strain the butter into a small bowl and immediately refrigerate it to set, about 30 minutes.
6. Place the butter in the bowl of a stand mixer and beat on medium-high speed until light and fluffy, about 5 minutes. Reduce the mixer speed to low and beat in the powdered sugar in small batches. Stir in the heavy cream, salt, and food coloring. Transfer the mixture to a pastry bag fitted with a round tip.
7. Collect the cookies:
   
   Pipe the buttercream onto 9 upside-down cookies, covering them in an even layer. Top with the remaining cookies and refrigerate the sandwiches until the buttercream has completely set, about 30 minutes.
8. Meanwhile, line a plate with parchment paper. Combine the chocolate and vegetable oil in a small saucepan and melt over medium heat until smooth. Carefully dip the cookies one at a time into the chocolate, coating them completely. Remove the cookies from the chocolate with a slotted spoon and transfer them to a parchment-lined plate. Once all the cookies are coated, refrigerate them until the chocolate hardens, about 10 minutes.