Heat the butter, milk and sugar in a large microwave safe bowl for 3 minutes, until simmering, then stir to mix.

Return the bowl to the microwave for another 2 minutes, until boiling.

Add the mini marshmallows to the bowl and heat for another 2 minutes, stirring after one minute.

Pour in the chocolate chips and stir until melted.

Fold in the chopped walnuts and vanilla extract.

Line a 9x13 baking dish with foil and grease with nonstick cooking spray. Spread the mixture evenly in the pan. Allow to cool to room temperature, then cut into squares and serve.