To make the ice cream: Freeze the work bowl of an ice cream maker according to the manufacturer's instructions., Place the milk, cream, and malted milk powder in a 2-quart saucepan set over medium heat. Bring to a simmer., While the milk mixture is heating, whisk together the egg yolks, sugars, and xanthan gum in a large heatproof bowl. Slowly add the hot milk, whisking constantly. Place a strainer over the saucepan and pour the cream/yolk mixture through it back into the pan., Reduce the heat to low and cook the base until it thickens slightly and you just begin to see steam come off the top. Remove from the heat, stir in the vanilla, and chill, stirring occasionally to prevent a skin from forming., To make the swirl: While the base is chilling combine the sugar, cocoa, corn syrup, malted milk powder, and half-and-half in a small saucepan set over medium heat. Cook until the mixture comes to a full boil, then reduce the heat to medium-low and simmer, stirring occasionally, for 3 minutes., Remove from the heat and add the butter and vanilla, stirring until the butter melts. Set aside to cool to room temperature., To freeze: Transfer the chilled base to the ice cream maker and freeze according to the manufacturer's instructions. Once the ice cream has begun to set, pour in 3/4 cup of the room-temperature swirl, for the last three or four turns of the machine. Save the rest for sundaes (store covered in the refrigerator for up to 3 months)., Transfer the ice cream to a container with an airtight lid and freeze for at least 2 hours before serving., Store, well covered, in the freezer for up to 3 months.