Classic Vanilla Slice

Name: Classic Vanilla Slice
Prep time: 20 min
Cook time: 40 min
Total time: 1 h
Cuisine: American
Ingredients: 2 sheets butter puff pastry (frozen (see notes)); 110 g caster sugar; 50 g corn starch (corn flour); 8 egg yolks (room temperature); 750 g whole milk (warmed); 14 g vanilla bean paste; 125 g unsalted butter (chilled and cubed); 50 g powdered sugar to dust