1/2 cup unsalted butter
3/4 cup packed light brown sugar
1/4 cup sugar
1 large egg
1 large egg yolk
1 tablespoon light corn syrup
1-1/2 teaspoons vanilla
1 cup all-purpose flour
1/4 teaspoon baking powder
1/8 teaspoon baking soda
1/8 teaspoon salt
2/3 cup shredded sweetened coconut
Preheat oven to 350 degrees F. Grease an 8-inch square baking pan lined with foil.
In a large heavy saucepan, melt butter over medium-low heat, then boil, stirring constantly until light golden brown, about 4 minutes, it should start to smell like butterscotch. Remove from heat quickly so it doesn’t get too browned and stir in sugars until well-blended. Let cool to room temperature.
Stir in egg, egg yolk, corn syrup, and vanilla. Add dry ingredients and coconut. Stir until combined. Scrape mixture into greased baking pan.
Bake until the top is golden brown and a toothpick inserted in the center comes out clean, 25-30 minutes. Cool completely in the pan on a rack.