Combine melted, cooled butter and sugars in a large bowl and stir until well-combined.
½ cup butter, melted and then cooled at least 10 minutes, ⅔ cup brown sugar, ¼ cup granulated sugar
Add egg and vanilla extract and stir well.
1 large egg, 1 teaspoon vanilla extract
In a separate, medium-sized bowl, whisk together flour, baking soda, baking powder, and salt.
1 ½ cups all purpose flour, ¼ teaspoon baking soda, ¼ teaspoon baking powder, ½ teaspoon salt
Gradually add flour mixture to wet ingredients, stirring until completely combined, then stir in chocolate chips.
1 cup mini semisweet chocolate chips
Cover bowl tightly with plastic wrap and transfer to the refrigerator to chill for at least 30 minutes and up to 5 days.
Once dough has chilled, preheat oven to 350F (175C) and line a baking sheet with parchment paper. Set aside.
Scoop dough by rounded 1 ½ teaspoon-sized scooponto prepared cookie sheets, placing at least 1″ apart. I like to roll my cookies between my palms into balls after scooping them so that they bake very uniform and round, but this is not necessary.
Bake on 350F (175C) for 6-8 minutes — cookies may still seem slightly soft in the centers, that is OK, they will cook completely on the cookie sheets. Don’t over-bake or your cookies will be too hard.