Preheat the oven to 180C/350F/Gas 4. Line an 8x8-inch square baking pan with foil or parchment paper. Set aside.
In a large bowl, whisk together the butter and sugar until combined. Add the egg, egg yolk, and vanilla, and whisk until smooth and combined.
Add the flour and salt, and whisk until completely combined.
Spoon half of the batter into the prepared pan. Place the whole KitKat bars on top, then spoon the remaining batter over the KitKats, spreading out evenly. Sprinkle with the crushed KitKat bars.
Bake for 35 - 40 minutes, or until the edges are browned and the centre is no longer jiggly - just set.
Allow to cool completely in the pan on a wire rack before cutting into squares.
Video
Notes
Blondies can be stored in an airtight container at room temperature or in the fridge for up to 1 week. They also freeze well for up to 3 months. Thaw overnight in the fridge before serving.