• Preheat the oven to 350°F. Grease a 9-inch pie plate (preferably deep dish) with butter, then sprinkle it all over with a little cacao powder, tapping out the excess.
• Put the milk in a medium saucepan and set over medium heat. Let it warm up just until it starts steaming, stirring every now and then.
• In a mixing bowl, whisk together the eggs, sugar, flour, tapioca starch, 2 tablespoons cacao powder, and salt until smooth.
• Once the milk is heated, gradually whisk about 1 cup of it into the chocolate-egg mixture, to temper the eggs. Then whisk the chocolate mixture into the sauce pan. Continue whisking constantly, with the pan over medium heat, until the mixture thickens to a pudding-like consistency, 2 to 5 minutes.
• Remove the pan from the heat and whisk in the 3 tbsp butter and the chocolate chunks, until smooth.
• Scrape the mixture into the prepared pan, smoothing the surface.

Topping:

• Whisk all ingredients together in a small bowl until evenly combined. Gently brush this over the surface of the pie.
• Transfer the pan to the oven and bake until the topping is browned, with the center set but still jiggly, 45 to 50 minutes. Cool completely. Then slice into wedges and serve at room temperature.