Preheat oven to 350°F. Place 6 ramekins into a larger baking dish, allowing a little space between the ramekins. Place a kettle of water on the stove and bring to a boil.
In a large bowl, whisk sugar, egg yolks and vanilla together until well blended. Sift cocoa powder over the egg mixture and whisk until smooth.
Heat the cream and buttermilk until small bubbles begin to form around the edge of the pot and steam is rising from the cream. Very slowly drizzle the warm cream mixture into the yolk mixture, whisking constantly until cream is fully incorporated. Add a few drops of food coloring to the mixture until desired color is reached and whisk to combine.
Strain the mixture through fine mesh sieve then evenly distribute the custard among the ramekins. Fill the pan with boiling water until it comes halfway up the sides of the ramekins.
Bake in preheated oven for 25-30 minutes until the custard is set but the center still jiggles slightly.
Remove from oven and remove the ramekins from the pan. Allow to rest at room temperature until cooled then refrigerate for at least 4 hours.
Just before serving, sprinkle a thin layer of sugar over the tops and caramelize using a kitchen torch or place under a hot broiler until a deep golden color is attained.