Preheat the oven to 350ºF.
Line an 8-inch square baking pan with parchment paper, allowing it to hang over the sides. Coat the paper with cooking spray.
In a stand mixer fitted with the whisk attachment, whisk together the melted butter with the brown sugar and Baker's sugar until smooth.
Add the eggs and vanilla.
Remove the bowl from the mixmaster and stir in the flour and salt just until moistened.
Stir in a ½ cup of the toffee bits.
Pour the batter into the prepared pan, smoothing the top.
Sprinkle the remaining ¼ cup of toffee bits over the top of the batter.
Bake for 35 to 40 minutes until a toothpick inserted comes out clean.
Let the pan cool completely then remove the baked blondies using the parchment overhang.
Cut them into 16 squares and serve.