In bowl of standing mixer fitted with paddle attachment, beat butter at medium speed until smooth and creamy. Beat in sugars until combined, about 45 seconds; mixture will look granular. Reduce speed to low and gradually beat in egg mixture until incorporated, about 45 seconds. Add melted chocolate and chips in steady stream and beat until combined, about 40 seconds. Scrape bottom and sides of bowl with rubber spatula. With mixer at low speed, add dry ingredients and mix until just combined. Do not over-beat.