Mix the milk, heavy cream, sugar, and vanilla extract in a medium saucepan over medium-to-low heat until the sugar is dissolved. Continue heating until the mixture is steamy and makes a slight sizzling noise when you move the pan. This is called scalding.

While the milk mixture heats to scalding, whisk together the egg yolks in a medium bowl until they’re light in color and slightly fluffy. Whisk the egg yolks and temper.

Gently stream about one-third of the hot milk mixture into the eggs while whisking continuously. This is called tempering. It’s important to whisk while streaming the hot milk. If you just pour in the hot milk and then whisk, you may get scrambled eggs.

Pour the egg and milk mixture into the rest of the milk mixture in the sauce pan and stir continuously on low heat with a heatproof spatula or flat ended wooden spoon. Make sure you scrape the bottom evenly while you continuously stir. The custard is thick enough when you can draw a line on the back of the spoon with your finger and the line retains its shape.

Melt the chocolate. Place the chocolate in a large heatproof bowl with a fine-mesh strainer on top. Pour the hot custard through the strainer into the chocolate. Stir until the chocolate is completely melted.

Cover with plastic wrap so that it’s directly touching the entire surface of the custard and none of it is exposed to air. This prevents a skin from developing. Transfer the container to an ice bath and let it cool for about 30 minutes to stop the cooking process. Transfer the container to the refrigerator. Chill until the custard is completely cold, at least 8 hours.

Dissolve the gelatin in the cold water. Pour the water into a small saucepan or microwave-safe container and evenly sprinkle the gelatin on top. Allow to sit until the gelatin appears to have absorbed as much water as it can, about 2 minutes. This is called blooming. Gently warm over low heat and stir until the gelatin is completely dissolved into the liquid, about 3 minutes.

Spike the custard with the cold chocolate liqueur, vodka, and gelatin mixture. Refrigerate the alcohol until completely cold. Do not speed up the process by putting it in the freezer, which may make the gelatin set up too much before it is added to the custard. Pour the gelatin into a medium bowl and whisk in the cold alcohol until combined. Do not attempt to skip this step by pouring the alcohol directly into the sauce pan or microwave-safe container with the gelatin. There might be enough residual heat to heat up the custard and prevent it from thickening in the ice cream maker. Pour the cold custard into a large bowl. Stream the alcohol and gelatin mixture through a fine-mesh strainer into the custard and whisk until thoroughly blended.

Churn the ice cream for at least 20 minutes. Pour the cold custard immediately into the ice cream maker and churn for at least 20 minutes, or as directed. Due to the alcohol content, you may wish to churn it longer to get the desired thickness. If you don’t want to serve the ice cream immediately, or if you want a firmer texture, transfer it to a freezer proof container and freeze for several hours before serving.