Step 1 In a medium, heavy-bottomed saucepan, combine the milk and cream and bring to a simmer over medium-high heat, then reduce the heat to medium.
Step 2 Meanwhile, in a small bowl, beat the egg yolks to combine. Slowly whisk about one-fourth cup of the milk mixture into the eggs to temper them, then transfer the eggs to the saucepan, whisking to combine.
Step 3Cook the custard, stirring frequently, until it is slightly thickened. Immediately add the chopped Mexican and bittersweet chocolates to the pan. Remove the pan from heat. Stir until the chocolate is melted and incorporated into the custard.
Step 4Strain the mixture into a large glass measuring cup or bowl. Ladle the chocolate into small bowls. Refrigerate the custards until set and chilled, at least 6 hours but preferably overnight.