Preheat oven to 375°F (191°C). Line a 9 x 13 inch pan with foil making sure that there is a 1-inch overhang on the longer sides for easy unmoulding. Lightly grease the foil.
Spread graham crackers into the bottom of the pan in a single layer. Break some of the squares in half to fit around the edges.
In a medium heavy-bottomed saucepan, melt butter over medium heat. Remove from heat. Add and stir in the Redpath® Dark Brown Sugar, heavy cream, and salt. Stir until mixture is thoroughly combined. Place saucepan back onto medium heat. Stir and bring mixture to a boil. Boil for 3 minutes without stirring. Pour mixture as evenly as possible over the graham crackers; do not worry if crackers are not completely covered.
Place baking pan into the preheated oven; about 8 minutes. The brown sugar mixture should be bubbling and spread over the tops of the graham crackers.
Take out of the oven and immediately sprinkle the tops of the graham crackers with the chopped chocolate. Wait about a minute before using an offset spatula or a heatproof spatula to evenly spread the melting chocolate over the crackers (see Chef’s Tips).
Set oven to broil. Wearing oven mitts, move the oven rack to the top third of the oven.
Evenly sprinkle the mini marshmallows over the melted chocolate, covering it completely.
Place pan back into the oven to toast marshmallows; approximately 1 to 3 minutes. Watch carefully to ensure marshmallows do not burn. Rotate pan as necessary. Remove from oven.
Let cool in pan; transfer to refrigerator to chill until chocolate is set; about 30 minutes. Remove using the foil overhangs and lay the slab onto a cutting board. Using a chef’s knife, cut into 3 x 5 inch or 3 x 6 inch pieces.