Preheat oven to 350 degrees F. Grease an 8x8 baking pan with 1 tbsp. of butter.
In the bowl of an electric mixer, whisk together melted butter and sugar until smooth, add the cognac and continue whisking on medium. Whisk in each egg one at a time on low speed and whisk until well combined.
Using a large rubber spatula, gently stir in flour, cocoa, baking soda and salt. Fold in Rollo chocolate caramels. Spread batter into the baking dish and bake for 25-30 minutes until set. 15 minutes into baking, remove dish from the oven and top with mini marshmallows, continue to bake until batter is settled.
Remove and let cool completely.
In the meantime work on the caramel sauce.
In a medium sauce pan over medium heat melt sugar until a brownish liquid forms, make sure to stir constantly during this process.
When sugar has completely melted, add the butter and stir until well combined.
Carefully while still stirring add the heavy cream, once all the cream has been added let the mixture boil without stirring for one minute.
Remove from heat, stir in salt, transfer to a mason jar (or any other container) and let it cool.