To make the dough
In the bowl of a stand mixer fitted with the dough hook, add the hot water, yeast, and ½ teaspoon cane sugar. Stir to combine and let sit for 5 minutes until bubbly.
In a small saucepan over medium heat, add the butter and milk. Stir until completely melted. Remove from the heat.
Add the flour, ¼ cup cane sugar, and sea salt to the mixing bowl in that order. Add the egg and the milk mixture and knead on low for 3 to 5 minutes, until a smooth dough forms. Place the dough in a greased bowl. Cover the bowl with plastic wrap and let rise for 1 to 3 hours, until doubled in size.
Preheat the oven to 350°F and grease a baking dish, either a 9-inch circle dish or an 11-by-7- inch dish.
To make the filling
In a small bowl, combine the cocoa and ¼ cup cane sugar. Set aside.
On a floured surface, roll out the dough to a rectangle that is about 12-by-16 inches. Using a spatula, spread the softened butter all over the rolled-out dough. Sprinkle the sugar and cocoa mixture on top of the butter. Starting at the longer side, fold the dough over and start to roll the dough until it is completely rolled into a log. Trim off the edges and cut the dough log into eight rolls.
Place the rolls in the prepared baking dish and bake for 35 minutes, or until golden brown. Allow to cool for 5 to 10 minutes.
To make the glaze
In a mixing bowl, add the powdered sugar and 3 tablespoons milk and whisk until you have a smooth glaze. Pour the glaze over the top of the warm sweet rolls.
Store in the fridge for up to 3 days.