Preheat oven to 350°F. Spray a 9x9” pan with cooking spray or line with foil and spray for easy removal and clean-up.
Cream butter and sugar with a hand or a stand mixer. Mix in egg, 2 teaspoons vanilla, and butter extract. Add baking soda, salt, cream of tartar and mix until combined. Slowly mix in flour and beat until smooth. Be careful not to over mix. Stir in 1/3 cup sprinkles.
Press batter into prepared pan. If it’s too sticky to handle, spray your hands with cooking spray.
Bake for about 17-22 minutes until the top just starts to get golden brown. (My bars to 18 minutes.) Cool completely before continuing.
Once bars are cool, add chocolate chips and sweetened condensed milk to a medium saucepan. Heat over low heat, stirring constantly, until melted and smooth. Remove from heat and stir in 1/2 teaspoon vanilla extract. Quickly pour and spread over cooled cookie bars. Quickly sprinkle more sprinkles on top and press lightly to adhere them to the surface. Let come to room temperature, then chill until set. Cut into squares.