In a heavy-bottomed saucepan of at least 2 quarts capacity, whisk together the granulated sugar, dutch processed cocoa powder, cornstarch, and kosher salt until even in colour.
Slowly drizzle about 1 cup of the milk into the dry ingredients while whisking vigorously. Continue whisking until it perfectly smooth and lump free. Whisk the rest of the milk in before placing the pan over medium heat.
Bring the mixture to a slow boil, stirring constantly, and cook only until thickened. Remove the pan from the heat. Stir in the chocolate chunks and continue stirring gently until the chocolate melts. Add the butter, vanilla and coffee extracts, and whisk until they are fully incorporated. Transfer to a large bowl or individual serving dishes. You can serve hot, warm, or cold.
If you would like to chill the pudding, and you dislike skin forming on the top of your pudding, lay a piece of plastic wrap directly on the surface. Otherwise, just refrigerate, loosely covered, until chilled through.