Melt 5 tbsp butter in a saucepan over medium-high heat. Once melted, remove from heat and whisk in 3/4 cup cold milk.
In the bowl of a stand mixer, combine flour, 1/2 cup granulated sugar, 1/3 cup black cocoa powder, active dry yeast, espresso powder, and 1 tsp kosher salt.
Place bowl on stand mixer and fit with the dough hook attachment. Mix flour mixture to combine, then add your melted butter and milk mixture and 1 large egg.
Mix for 5-7 minutes on medium-high speed until the dough forms a strong, elastic ball. Remove from bowl and knead by hand for another 5 minutes.
Place back in bowl and cover with plastic wrap. Proof in a warm spot for 2-3 hours, or until doubled in size.
In the meantime, heat 5 tbsp butter in a medium saucepan until fully melted and beginning to brown. Add your light cream and stir to combine, cooking for another 2-3 minutes.
Remove from heat and add your chopped dark chocolate, ground cinnamon, and sea salt. Then, add 1/4 cup black cocoa powder, powdered sugar, and whisk to combine.
When your chocolate dough has doubled in size, remove from bowl and pat into a disc. Roll into a large rectangle, about 18” x 36”.
Spread your chocolate mixture evenly over the top with an offset spatula. Then, roll your cinnamon rolls into a tight log, starting at the short end of the dough.
Trim the edges of the dough log to remove the imperfect offcuts. Then, cut your log into 8 equal buns.
Butter or grease a 9x9” square baking pan and place your buns inside, spaced evenly. Cover with plastic wrap and proof for another 1-2 hours until puffy and slightly larger in size.
Preheat oven to 350°F. Remove plastic cover and bake buns for 22-26 minutes until fully risen and firm to the touch.
Combine 3/4 cup granulated sugar with 1/4 cup water and cream of tartar in a medium saucepan. Cook over medium high heat until bubbly and lightly boiling.
Meanwhile, whip your egg whites and vanilla in a stand mixer or by hand until soft peaks form. Then, carefully pour your hot sugar syrup into the egg whites while whisking on high speed until it holds stiff peaks.
Frost your cinnamon rolls with a generous amount of the marshmallow fluff. Gently brûlée with a blow torch.