Melt the unsalted butter in a small skillet over medium heat. Add the Grape Nuts cereal and toast for 3-5 minutes, stirring frequently, until fragrant and slightly darkened. Remove from heat and let cool completely.

In a large mixing bowl, pour in the cold heavy whipping cream. Using an electric hand mixer or a stand mixer with a whisk attachment, beat on medium-high speed until stiff peaks form, about 3-4 minutes.

In a separate medium bowl, whisk together the sweetened condensed milk, vanilla extract, and sea salt until smooth and well combined.

Gently fold the condensed milk mixture into the whipped cream using a rubber spatula. Use a light sweeping motion to ensure you do not deflate the whipped cream, folding just until no streaks remain.

Sprinkle the cooled, toasted Grape Nuts over the cream mixture. Gently fold them in until the cereal is evenly distributed throughout the base.

Transfer the ice cream base into a 9x5-inch metal loaf pan or a freezer-safe container. Smooth the top with a spatula and cover tightly with plastic wrap or a snug-fitting lid.

Freeze for at least 6 hours, or ideally overnight, until completely firm. Let the ice cream sit at room temperature for 5 minutes before scooping to soften slightly, and serve in chilled bowls or crispy waffle cones.