In a heavy-bottom saucepan, combine the milk and cream. Place over medium-low heat and cook, stirring occasionally so a skin doesn’t form, until tiny bubbles start to form around the edges and the mixture reaches a temperature of 170°F. Turn off the heat and whisk in the cocoa powder and cinnamon. Add the chopped chocolate, and stir or whisk until the chocolate is completely melted and the mixture is smooth.
Meanwhile, in a medium heat-proof bowl, whisk the egg yolks until smooth. Gradually whisk in the sugar until it is well incorporated and the mixture is thick and pale yellow. Temper the egg yolks by very slowly pouring in the hot milk mixture while whisking continuously. Return the custard to the saucepan and place over low heat. Cook, stirring frequently with a wooden spoon, until the custard is thick enough to coat the back of the spoon and it reaches a temperature of 185°F. Do not bring to a boil.
Pour the mixture through a fine-mesh strainer into a clean bowl and let cool to room temperature, stirring every 5 minutes or so. To cool the custard quickly, make an ice bath by filling a large bowl with ice and water and placing the bowl with the custard in it; stir the custard until cooled. Once completely cooled, cover and refrigerate until very cold, at least 4 hours or overnight.
Based on a recipe from The Ciao Bella Book of Gelato and Sorbetto