Step 1

Melt the Chocolate: In a medium bowl (large enough to hold the entire ice cream mixture), combine the milk chocolate and bittersweet chocolate. Melt the chocolate in the microwave in 20-second intervals, stirring in between, until about 75% melted. Stir, allowing the residual heat in the bowl to melt the chocolate completely. (Alternatively, place the chocolate in a heatproof bowl and place over a saucepan containing about 1 inch of barely simmering water. Stir until the chocolate is melted and the mixture is smooth.) Add the cocoa powder and salt and stir until completely smooth.

Step 2

In a small bowl, stir together the milk powder and sugar. Set aside.

Step 3

In another small bowl, combine the cornstarch with 2 tablespoons of cold milk and stir until completely smooth. Set aside.

Step 4

In a medium saucepan over medium-high heat, cook the remaining 2¼ cups milk, the cream, and corn syrup, whisking occasionally to prevent scorching, until it comes to a rolling boil.

Step 5

Whisk the milk powder and sugar mixture into the pot, reduce the heat to a low simmer, and continue cooking for 2 minutes, whisking frequently to prevent scorching. Add the cornstarch mixture and continue cooking and whisking for 1 minute more.

Step 6

Pour about one-third of the hot milk mixture into the melted chocolate; whisk until completely smooth. Gradually pour in the remaining milk mixture, stopping to whisk until smooth as you go, until fully incorporated (adding the milk mixture slowly prevents flecks of unmixed chocolate in the ice cream, which turn grainy when frozen).

Step 7

Fill a large bowl with crushed ice and a bit of cold water, a few inches deep. Next, place the warm chocolate ice cream base into the ice bath, stirring occasionally until it reaches a cool room temperature, 15 to 20 minutes. (These ice baths can be a little wobbly; be sure your bowl is stable. It's fine to use less ice and water if necessary.)

Step 8

Remove the chocolate ice cream base from the ice bath, cover with plastic wrap, and refrigerate for at least 4 hours or overnight. (The colder the mixture is when you churn it, the better.)

Step 9

Pour the ice cream mixture into the bowl of an ice cream machine and churn according to the manufacturer's instructions. The ice cream is ready when it reaches a soft-serve consistency and holds its shape; it should take 20 to 30 minutes, depending on the machine.

Step 10

Immediately transfer the ice cream to a container with an airtight lid. Press plastic wrap on the surface of the ice cream to prevent ice crystals from forming, cover, and freeze until it hardens completely, 6 to 12 hours. Scoop and serve.