Line an8x8 inch panwith parchment paper and lightly spray with cooking spray. Set aside.
Heat a large pot on low heat and melt the butter. Then, stir the marshmallows into the melted butter until they are smooth and completely melted.
Remove the pot from the heat and add in the crushed Brownie Brittle. Stir until evenly mixed.
Press the mixture into the prepared pan evenly. You can spray your hands with a little bit of cooking spray if the mixture sticks to them.
In a small, microwave safe bowl, melt the caramels and heavy cream in the microwave. I find it works best to microwave for 30 seconds at a time, and stir between each session until the caramel is totally melted. Stir well to combine the caramel and cream.
Pour the caramel mixture over top of the marshmallow treats and let cool and harden.