Place the butter and salt into a large pot over medium-low heat and stir until the butter becomes fully melted and glossy.

Pour in the sweetened condensed milk and stir gently until smooth and creamy.

Add the mini marshmallows and continue stirring slowly as they melt into a thick, silky mixture with no visible lumps remaining.

Remove the pot from the heat immediately once the marshmallows are melted to prevent the treats from becoming hard.

Stir in the vanilla extract for a warm bakery-style aroma.

Fold in the crispy rice cereal using a rubber spatula until every piece is coated in the marshmallow mixture.

Add the extra mini marshmallows and gently mix so pockets of soft marshmallow remain throughout the treats.

Transfer the mixture into the prepared pan and lightly press it into an even layer using greased hands or a buttered spatula.

Avoid compacting the mixture too firmly so the bars stay soft and airy instead of dense.

Let the treats cool at room temperature until set but still slightly soft in the center.

Slice into squares or rectangles with a buttered knife and serve fresh.