01 -
Preheat oven to 175°C. Line a 20 x 20 cm light-coloured metal pan with parchment or foil, and coat with nonstick spray.
02 -
In a large bowl, mix hot melted butter with dark brown sugar using a rubber spatula. Allow mixture to cool slightly.
03 -
Incorporate eggs, egg yolk, and vanilla extract into butter mixture, stirring until well combined.
04 -
Stir in fine salt and all-purpose flour until a uniform blondie batter forms.
05 -
Evenly spread half the batter in the prepared pan. Bake in preheated oven for 15 minutes, until the surface just begins to set.
06 -
In a heatproof vessel, microwave cookie butter and sweetened condensed milk in 20-second intervals, stirring between bursts, until warmed and emulsified.
07 -
Flatten portions of the cookie butter mixture with your palms, arranging in an even layer over the par-baked blondie base.
08 -
Spread the remaining blondie batter gently to fully cover the cookie butter layer.
09 -
Bake at 175°C for an additional 30 minutes, or until golden and set throughout.
10 -
Allow to cool completely in the pan before cutting into squares for serving.