Begin by preheating your oven to 350°F (175°C). This ensures that your Candy Topped Toffee Squares will bake evenly.
Grease a 9x13 inch baking pan with a little butter or line it with parchment paper for easy removal.
In a large mixing bowl, combine 1 cup of softened butter and 1 cup of dark brown sugar. Use an electric mixer to beat the mixture until it is light and fluffy, about 3-5 minutes.
Add in 1 egg yolk and 1 teaspoon of vanilla extract. Beat until well combined, scraping down the sides of the bowl as needed.
In a separate bowl, whisk together 2 cups of all-purpose flour and a dash of salt. Gradually add this mixture to the butter mixture, stirring until just combined.
Using a spatula, spread the batter evenly into the prepared baking pan. Make sure it's smoothed out to the edges for even baking.
Place the pan in the preheated oven and bake for about 20-25 minutes, or until the edges are golden brown. The center may look a bit soft; that’s perfect!
As soon as you take the pan out of the oven, sprinkle your choice of candy toppings over the warm toffee base. This could include chocolate chips, nuts, or colorful candies. The heat will melt the candies slightly, allowing them to stick to the squares.
Allow the Candy Topped Toffee Squares to cool completely in the pan. Once cooled, cut them into squares and enjoy!