In a medium saucepan, combine the heavy cream, whole milk, 1/2 cup of the sugar, salt, and ground nutmeg. Heat over medium heat until it just begins to simmer, then remove from the heat.

In a separate medium bowl, whisk the egg yolks and the remaining 1/4 cup of sugar together until pale and slightly thickened, about 2 minutes.

Gradually pour about 1 cup of the hot cream mixture into the egg yolks while whisking constantly to temper the eggs and prevent them from scrambling.

Pour the tempered egg mixture back into the saucepan with the rest of the cream and return to medium-low heat. Cook, stirring constantly with a wooden spoon or spatula, until the custard thickens enough to coat the back of the spoon, about 5 to 7 minutes.

Remove the saucepan from the heat and stir in the vanilla extract. Pour the custard through a fine-mesh strainer into a clean bowl to remove any cooked egg bits.

Stir the Grape-Nuts cereal directly into the warm custard so the cereal can soften slightly and infuse the base with its malty flavor. Cover the bowl with plastic wrap pressed directly against the surface of the custard and chill in the refrigerator for at least 4 hours, or overnight.

Once fully chilled, pour the mixture into your ice cream maker and churn according to the manufacturer's instructions until it reaches a soft-serve consistency.

Transfer the churned ice cream to a freezer-safe container, smooth the top, and freeze for at least 4 hours until firm before serving in chilled bowls or waffle cones.