Beat the butter, ½ cup brown sugar and the sugar in a large bowl with a mixer on medium speed until creamy, about 3 minutes. Add the egg yolks; beat until light, about 2 minutes. Blend in flour, salt, and baking powder until mixture comes together but is still crumbly.
Pat into an ungreased 13x9 inch baking pan. Top the dough with chocolate chips, pressing them down into the dough; set aside.
Beat the egg whites in a medium bowl with a mixer on medium speed until stiff; add the remaining 1 cup of brown sugar. Beat until well combined; spread over chocolate chips.
Bake until a tester tests clean, about 25 minutes. Cool before cutting.
Recipe by Opsreys Journeys at https://www.ospreysjourneys.com/?p=2839