Preheat your oven to 350°F (175°C).
Lightly grease your 9x13-inch baking pan with cooking spray or line it with parchment paper for easy removal later.
In a mixing bowl, combine the all-purpose flour and powdered sugar. Cut the cold butter into small pieces and add it to the mixture. Using a pastry cutter or your fingers, work the butter into the flour until the mixture resembles coarse crumbs. Press this mixture firmly into the bottom of the prepared baking pan to form an even crust.
Place the crust in the preheated oven and bake for about 15 minutes or until lightly golden.
While the crust bakes, in a separate bowl, whisk together the sweetened condensed milk, egg, and vanilla until well combined. Stir in the toffee bits and sliced almonds, ensuring everything is evenly distributed.
Once the crust is done, remove it from the oven and pour the toffee filling over the hot crust. Spread it evenly with a spatula.
Return the pan to the oven and bake for an additional 20-25 minutes, or until the filling is set and the top is golden brown.
Allow the bars to cool in the pan on a wire rack. Once cooled, use a sharp knife to cut them into squares or rectangles. Serve and enjoy!