Step 1: Prepare the Dough
In a stand mixer fitted with the paddle attachment, combine the warm milk, warm water, instant yeast, sugar, flour, 2 tablespoons of melted butter, and salt. Mix until just combined. Switch to the dough hook and knead on low speed for 6-8 minutes until smooth and elastic. Cover and rest for 15 minutes.

Step 2: Make the Chocolate Filling
While the dough rests, combine the cocoa powder, heavy cream, chocolate chips, sugar, butter, vanilla extract, and salt in a medium saucepan. Whisk continually over low-medium heat until the mixture is smooth and blended. Remove from heat and let cool.

Step 3: Roll and Fill
Preheat oven to 350 degrees Fahrenheit and coat a 9.5-inch round pan with baking spray. Roll the dough into a 10-inch by 16-inch rectangle. Brush with the remaining 2 tablespoons of melted butter, leaving a 1/2-inch border. Spread the cooled chocolate filling over the butter, then sprinkle evenly with graham cracker crumbs.

Step 4: Cut and Bake
Roll the dough tightly starting from the long side and pinch the seams to seal. Cut into 8-10 equal rolls and place them cut-side down in the prepared pan. Bake for 25-28 minutes until light golden brown. Transfer to a wire rack to cool for 10 minutes.

Step 5: Create the Meringue
In a double boiler over simmering water, whisk the egg whites and sugar until the sugar is fully dissolved (about 5-6 minutes). Transfer to a stand mixer with the whisk attachment. Whip on high until soft peaks form, then add cream of tartar, salt, and vanilla. Continue whipping until stiff, glossy peaks form (about 10 minutes total).

Step 6: Top and Serve
Generously spread the marshmallow meringue over the warm rolls. For the full s’mores experience, toast the meringue lightly using a kitchen blowtorch or by placing it briefly under the oven broiler (watch closely to prevent burning).