Preheat oven to 350 degrees. In a large bowl, using an electric mixer, beat butter, brown sugar, and salt on medium-high until light and fluffy, about 3 minutes. With mixer on low, add flour in three additions and beat until combined (dough will be crumbly). Line an 8-inch square baking dish with foil and spray with non-stick cooking spray. Press dough evenly into baking dish. Bake until golden brown and firm, 30 to 35 minutes.

Scatter chocolate chips on top of shortbread. Bake until soft, 1 minute. With the back of a spoon, spread chocolate evenly over shortbread. Scatter candies over top. Gently press in. Let cool on a wire rack for 30 minutes. Refrigerate briefly to set chocolate. Remove foil/shortbread from baking dish, then cut into 16 bars.