Preheat the oven to 350°F (175°C). Line a 9×13-inch baking pan with aluminum foil, allowing the foil to overhang on the sides for easy removal. Lightly coat the foil with nonstick cooking spray and set aside.

In a small bowl, whisk together the flour, cornstarch, baking powder, and salt. Set the dry ingredients aside.

In a large mixing bowl, beat the softened butter with an electric mixer on medium speed for about 30 seconds, just until smooth. Add the granulated sugar and brown sugar, and continue beating on medium-high speed until the mixture becomes light and fluffy, about 2–3 minutes. Add the egg yolk and vanilla extract, and beat until fully incorporated.

Gradually add the flour mixture to the butter mixture and mix on low speed until a soft, crumbly dough forms.

Press the dough evenly into the bottom of the prepared pan. Bake for 10 to 12 minutes, or until the edges are lightly golden.

While the crust is baking, place the sweetened flaked coconut in a large bowl. Pour in the caramel sauce and stir until the coconut is evenly coated and the mixture is fully combined.

Remove the crust from the oven and immediately sprinkle 1½ cups of semisweet chocolate chips over the hot crust. Let sit for 5 minutes to allow the chocolate to soften, then gently spread the melted chocolate into an even layer using an offset spatula or the back of a spoon.

Carefully spoon and spread the caramel coconut mixture evenly over the chocolate layer, pressing it down lightly to help it adhere.

Return the pan to the oven and bake for an additional 10 minutes. Remove from the oven and let the bars cool completely in the pan on a wire rack.

In a small microwave-safe bowl, melt the remaining chocolate chips in 20-second intervals, stirring after each burst, until completely smooth and pourable. Drizzle the melted chocolate over the cooled coconut layer in a decorative pattern.

Cover and refrigerate the bars for at least 1 hour or until fully set. Once chilled, use the foil overhang to lift the bars from the pan. Place on a cutting board and cut into squares or bars using a sharp knife.