In a small bowl, microwave ¼ cup of the chocolate chips until melted. Start with 1 minute on high, then remove the bowl from the microwave and stir well. Continue heating in 30-second intervals (stirring between intervals) until the chocolate is melted.
¾ cup semisweet chocolate chips
In a large bowl, using a hand mixer, beat the butter, brown sugar, granulated sugar, vanilla extract, and melted chocolate together until well mixed, about 2 minutes.
½ cup unsalted butter, ¾ cup brown sugar, ¼ cup granulated sugar, 1 teaspoon pure vanilla extract
Beat in the heat-treated flour, cocoa powder, salt, and milk until fully combined, about 1 minute. Use a rubber spatula to scrape down the sides of the bowl.
½ cup unsweetened cocoa powder, ¼ teaspoon kosher salt, 2 tablespoons milk
Stir in the remaining ½ cup of chocolate chips.
(Optional) Roll the brownie batter into equally-sized balls for an on-the-go treat.