Preheat your oven to 350°F. Line a 9x13-inch baking pan with foil and lightly coat it with non-stick cooking spray.

In the bowl of a stand mixer with a paddle attachment (or with a hand mixer), combine softened butter, flour, salt, and powdered sugar. Mix until the ingredients are well-blended and the mixture is smooth. Fold in 1/4 cup of the chopped almonds. The mixture will be slightly sticky.

Transfer the shortbread mixture to the prepared pan. Using hands lightly sprayed with cooking spray, press the dough into an even, thin layer across the bottom of the pan. Bake this shortbread layer for 15 minutes.

While the crust bakes, prepare the topping by stirring together the sweetened condensed milk, egg, and vanilla extract in a separate bowl. Mix in the remaining 1/2 cup of chopped almonds, the mini chocolate chips, and toffee bits until evenly distributed.

Once the crust has baked for 15 minutes, gently pour the topping mixture over the hot crust. Spread it evenly, then return the pan to the oven. Bake for an additional 18 minutes, or until the edges of the topping start to turn golden brown.

Allow the bars to cool completely in the pan. Once cool, lift the bars out using the foil lining and cut into squares. Store in an airtight container for up to 3 days, or freeze for up to one month.