Grease the pan. For easy removal of the bars, place parchment paper on the bottom and 2 sides of the pan. Grease again and then set aside. Preheat oven to 350°F. Melt the butter and set aside as you prep the rest of the ingredients.
In a large bowl, add melted and cooled butter, brown sugar, and white sugar. Whisk until well combined.
3/4 cups butter, 3/4 cups light brown sugar, 1/4 cup white sugar
Add in egg, yolk, and vanilla. Continue whisking.
1 egg, 1 egg yolk, 1 tbsp vanilla
Add in the flour, baking powder, and salt. Using a rubber spatula, gently fold it in until a soft batter forms. It will be runny.
2 cups all-purpose flour, 1/2 tsp baking powder, 1/2 tsp salt
Toss in the semi-sweet and mini chocolate chips and mix in a few times.
1/2 cup semi-sweet chocolate chips, 1/2 cup mini chocolate chips
Pour into the prepared pan and spread out evenly. Top with extra chocolate.
Bake for 22-25 minutes or until the blondies are a light golden colour. The bars continue to bake as they cool in the hot pan, so it's okay if they're underbaked when removed from the oven.
Let the cookie bars cool completely before slicing, and enjoy! I personally love to have them with a scoop of vanilla ice cream.