Prepare an ice bath by filling a large bowl with ice cubes and 1 to 2 cups of water. Place a medium bowl fitted with a fine strainer inside the ice bath.

In a medium saucepan, combine the milk, cream, 1/2 cup sugar, milk powder, salt and vanilla. Set over medium heat, stirring occasionally, until the mixture is warm and begins to steam, about 5 minutes.

In a medium bowl, whisk together the egg yolks and the remaining 1/4 cup sugar. Carefully whisk half of the warm milk mixture into the egg yolks, one ladleful at a time, until the egg mixture is warmed. Whisk the egg-milk mixture back into the saucepan. Cook the mixture over medium heat, stirring constantly with a wooden spoon, until the mixture is thick enough to coat the back of the spoon and registers around 175 degrees F on an instant-read thermometer, about 5 to 7 minutes. Be careful not to boil the mixture.

Immediately strain the mixture through the fine strainer into the prepared ice bath. Cool the custard in the ice bath until it reaches room temperature, stirring often. Press plastic wrap against the surface of the custard and refrigerate until chilled, about 4 hours or up to 1 day.

Pour the chilled mixture into an ice cream maker and freeze according to the manufacture's directions. Transfer the ice cream to an airtight container, press plastic wrap against the surface of the ice cream, and freeze until firm and the flavor is ripened, at least 2 hours.

Preheat the oven to 350 degrees F. Line large baking sheets with parchment paper or silicon baking mats.

In a medium bowl, sift together the flour, cocoa powder, milk powder, baking soda and salt.

In a large bowl, use an electric mixer to beat the butter and sugars on medium-high speed until well combined and smooth. Beat in the egg, vanilla, and sour cream. On low speed, add the flour mixture and beat until combined.

Roll dough into 2 tablespoon-sized balls and place on the prepared baking sheets. Flatten with the palm of your hand.

Bake for about 10 to 12 minutes, or until the cookies are set. Let cool on a baking sheet for 5 minutes before removing to wire racks to cool completely. Freeze the cookies until frozen, at least 1 hour. Cookies can be stored in airtight containers in the freezer for up to 1 month.

Crushed malted milk ball candies (about 1 cup), for rolling.

To assemble, place the crushed malted milk ball candied in a shallow dish. Top one cookie with a coop of ice cream. Place another cookie on top of the ice cream and gently press down to form a sandwich. Roll the edges in the crushed malted milk ball candies and repeat for the remaining cookies. Freeze for at least 1 hour before serving.