Design Eat Repeat

0 Preheat oven to 350°F.
17 Tip before starting! I will note that this is a thicker dough that is easiest to make with a stand mixer. If you only have a handheld mixer, you may just need to use a spatula to help mix the flour in.  
1 In a large bowl, use an electric mixer on medium-high speed to cream softened unsalted butter ([adjustable]¾ cup, 1 ½ sticks), light brown sugar (¾ cup), and granulated sugar (¾ cup)[/adjustable] for 2 minutes until light and fluffy.
2 Add the eggs [adjustable](2 large), vanilla extract (2 teaspoons), and light corn syrup (3 tablespoons)[/adjustable] and mix on low speed until combined. Scrape down the sides of the bowl so everything can combine.
3 Add in baking powder ([adjustable]1 teaspoon) and salt (½ teaspoon)[/adjustable] and mix for another 5-10 seconds until combined.
4 Lastly, add dark cocoa powder [adjustable](¾ cup) and all-purpose flour (2 ¼ cups)[/adjustable] and mix on medium until combined.
5 Spray a light-colored metal 9×13 pan with cooking spray, then line pan with parchment paper, leaving a 2 inch overhang on each long side so that you will be able to lift the bars out of the pan.
15 Drop the dough into 1-tablespoon sized chunks into the prepared pan and spread until smooth on top.Tip: This dough is a little sticky, so to help spread, I recommend spraying your hands with cookie spray or use place another piece of parchment paper on top and continue pressing down.
6 Bake bars at 350°F for 18-22 minutes, or until the center no longer looks wet. The center may look a tiny bit underdone but the bars will continue to bake in the hot pan when cooling.Allow to fully cool before decorating.
To Make Chocolate Coating:
8 Add chocolate chips [adjustable](1 cup) and unsalted butter (¼ cup, i.e. ½ stick)[/adjustable] to a small to medium-sized microwafe-safe bowl. Heat on 50% power for 30 second increments until full melted, stirring in between (even if nothing is melted the first few rounds, still stir it!). Microwaving on 50% power is important to melt the chocolate slowly to prevent from the chocolate seizing or burning.
9 Once melted, add in the [adjustable]¼ [/adjustable]teaspoon peppermint extract and stir. Before spreading, set aside about 2 tablespoons of chocolate for the drizzle.Note that this coating isn't as liquidy as a ganache, it's a little firmer so that it hardens up well. That said, if your mixture looks too thick to spread, you can add 1 tablespoon of melted butter to help thin.
10 Spread melted chocolate over cooled cookie bars until tops and sides are covered. Spoon the [adjustable]2[/adjustable] tablespoons of reserved chocolate in a piping bag or ziploc bag with a tiny corner cut off, then drizzle over top for added presentation.
16 Allow chocolate to harden before cutting; it's best to refrigerate or freeze for 30 minutes to help set. To get clean slices, fill a tall cup with hot water. In between cuts, dip the knife into the water and use a paper towel to dry it off. When cutting, don't "saw" the bars but rather press straight down and gently lift the knife.