Whoopie pies — those traditional New England treats — are like little cakes in the shape of cookies, and they typically sandwich a thick layer of frosting. In this recipe, chocolate whoopie pies sandwich fresh cherry ice cream to make a frozen version of Black Forest cake. You don’t have to be from New England to enjoy these sandwiches.
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, ½ cup sugar, and salt. 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 ¼ cup sugar. Carefully whisk 1/2 of the warm milk mixture into the egg yolks, 1 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, until the mixture is thick enough to coat the back of the spoon and registers around 175 degrees, about 5 to 7 minutes. Be careful not to boil the mixture.
Immediately remove from the heat and stir in the cherries. Let the mixture sit for 10 to 15 minutes, or until slightly cooled. With an immersion blender, food processor, or blender, carefully purée the mixture until smooth. Add in the vanilla. 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. Freeze according to the manufacturer’s directions. Transfer the ice cream to an airtight container, press plastic wrap against the ice cream surface, and freeze until it is firm and the flavor is ripened, at least 2 hours.
Preheat the oven to 375 degrees. Line large baking sheets with parchment paper or silicone baking mats.
In a medium bowl, sift together the flour, cocoa powder, baking soda, and salt.
In a large bowl, use an electric mixer to beat the butter and sugars on medium-high speed until light and fluffy, about 3 minutes. Add the egg and beat for an additional 2 minutes. Add in the vanilla. On low speed, gradually add 1/2 of the flour mixture. Gradually add the milk and beat until combined. Add the remaining flour mixture and beat until combined.
Drop 2 tablespoon–sized balls of dough about 2 ½ inches apart on prepared baking sheets.
Bake for about 10 minutes, or until the whoopie pie halves are set and spring back when gently pressed. Do not overbake. Let the whoopie pie halves cool on pans for 5 minutes, then remove to wire racks to cool completely. Freeze the whoopie pie halves until frozen, at least 1 hour. Whoopie pie halves can be stored in airtight containers in the freezer for up to 1 month
To assemble, top 1 whoopie pie half with a scoop of ice cream. Place another whoopie pie half on top of the ice cream and gently press down to form a whoopie pie sandwich. Repeat for the remaining whoopie pies. Freeze for at least 1 hour before serving.