While blondies are baking, in a 2 quart saucepan melt butter then add brown sugar. Stir until dissolved.
Next, add corn syrup and continue to stir.
Slowly add condensed milk, stirring constantly.
Cook over medium heat stirring constantly. Once caramel starts to boil, continue cooking until caramel reaches 230°
When the caramel reaches 230° remove from heat, add vanilla, stir, then add cashews. Stir.
Once blondies have been removed from oven, spread caramel mixture evenly over the top.