½cupchocolate chunks, plus extra to press into top of the dough before baking
Make the dough: Place the butter and mint in a small saucepan over medium heat. Melt the butter, swirling occasionally. After about 2 to 3 minutes, when you can smell the mint, remove from heat and let the butter continue to steep for 30 minutes.
Strain the butter through a fine mesh sieve into a medium bowl, pressing the leaves to extract all of the butter from the leaves. Allow butter to cool to a semi-soft texture (it will help the creaming process if the butter isn’t too hard or still liquid).
Add the sugars to the butter and beat with a hand mixer until everything is light and creamy, about 3 to 5 minutes. Add the egg and vanilla extract and beat for an additional minute.
In a small bowl, sift the flour and cocoa powder together to remove any lumps, then whisk in the baking soda and salt.
Add the dry ingredients to the butter mixture and beat a low speed until just combined, stopping to scrape the bowl as necessary. Fold in the chocolate chunks.
Remove the cookie dough from the bowl, wrap in plastic wrap, and press flat into a disk. Chill for at least 1 hour (or up to 3 days)
Divide the dough into 12 equal portions (I use a small cookie scoop), rolling into 12 balls. Space six balls on each cookie sheet (the cookies can spread considerably). Press a few extra reserved chocolate chunks in the top of each cookie.
Bake for 11 to 12 minutes for cookies, rotating the pans half-way through.
Let cool on the baking sheet for 2 minutes, and then use a large spatula to move to a cooling rack and cool completely.
Press the dough into the prepared baking pan as evenly as possible (use plastic wrap on the dough and a small pie roller if possible). Press some extra chocolate chunks in the top of the dough.
Bake bars for 25 to 30 minutes. Allow to cool in the pan for 15 minutes, then use the parchment paper overhang to lift bars onto a cooling rack to cool completely.
Cut into 48 approximately 1- x 2-inch bars.
If you don’t want to deal with infusing butter with mint (although it’s so simple!), you can substitute ½ teaspoon peppermint extract, adding with the vanilla.For the pictures, I made one batch of dough to bake half as cookies and half as bars, so I used one baking sheet and a 9- x 9-inch baking pan. These bars baked for 20 minutes.