Preheat the oven to 350 degrees F. Line 2 cookie sheets with parchment paper.
Melt the butter over medium heat in a large saucepan. Remove from heat and add the sugar. Whisk until smooth and let cool for 5 minutes.
Mix the flour, baking powder, and salt together and set aside.
Whisk the egg and vanilla into the slightly cooled butter mixture, then add the flour mixture. Stir with a rubber scraper until mixed, then (after making sure the mixture is not so warm it will melt the chips) stir in chips.
Drop dough by heaping tablespoons onto baking sheets spacing 1 1/2 inches apart.
Bake 1 sheet at a time on center rack for about 12 minutes.
Carefully transfer to a cooling rack and let cool completely.
Melt the chocolate bar in the microwave using 50% power and stirring every 30 seconds. Drizzle over the cookies, then chill the cookies just until the chocolate has set.
I had both types of brown sugar in the pantry so I used a little dark for added molasses flavor. Feel free to just use 100 grams of light brown sugar.