2 1/2 c. all-purpose flour
1 tbsp. cornstarch
1 1/4 tsp. baking powder
3/4 tsp. kosher salt
1 c. unsalted butter, softened, plus more for greasing the skillet
1 1/4 c. light brown sugar
2 large eggs
2 tsp. vanilla extract
1 1/2 c. semisweet chocolate chips, plus more to top the cookie
1/2 c. toffee bits
1/2 c. unsalted butter, softened
1 3/4 c. powdered sugar
1/4 c. unsweetened cocoa powder
Pinch of kosher salt
2 tsp. vanilla extract
2 tbsp. whole milk, divided
Multicolored sprinkles, for decorating