Heat 11 tbsp butter in a skillet over medium-high heat until dark golden and nutty, 3-5 mins.
Transfer to a bowl and stir in remaining 4 tbsp butter until melted. Let rest for 15 mins.
Preheat oven to 350°F and line two baking sheets with parchment paper.
Mix 6 tbsp granulated sugar with 1/4 cup brown sugar in a shallow dish.
Whisk flour, baking soda, and baking powder in a medium bowl.
Add 1.5 cups brown sugar and salt to cooled butter. Stir smooth, 30 secs.
Add egg, egg yolk, and vanilla to the butter mixture. Mix 30 secs.
Add flour mixture and mix until combined, 1 min.
Divide dough into 24 balls (2 tbsp each), coat in sugar mixture.
Arrange 12 balls per sheet, 2 inches apart.
Bake one sheet at a time until edges set, centers soft, 12-14 mins, rotating halfway.
Cool on baking sheet for 5 mins. Transfer to wire rack to cool completely.
Serve and enjoy!