In a small skillet over medium heat, melt the butter and honey together. Add the Grape-Nuts cereal and toast for 3-4 minutes, stirring constantly until fragrant and slightly darkened. Remove from heat, spread onto a plate to cool completely, and sprinkle with sea salt.

In a medium saucepan, whisk together the heavy cream, whole milk, and half of the sugar. Heat over medium until it just begins to simmer, then remove from heat.

In a separate bowl, whisk the egg yolks with the remaining sugar until pale and slightly thickened.

Slowly pour about a cup of the hot cream mixture into the egg yolks while whisking constantly to temper the eggs.

Pour the egg mixture back into the saucepan with the remaining cream. Cook over low heat, stirring constantly with a wooden spoon, until the custard thickens enough to coat the back of the spoon (about 170°F or 77°C). Do not let it boil.

Strain the custard through a fine-mesh sieve into a clean bowl. Stir in the vanilla extract.

Cover the bowl with plastic wrap directly touching the surface of the custard to prevent a skin from forming. Refrigerate for at least 4 hours or overnight until thoroughly chilled.

Churn the custard in an ice cream maker according to the manufacturer's instructions.

In the last minute of churning, pour in the cooled, honey-roasted Grape-Nuts.

Transfer the soft ice cream to a freezer-safe container and freeze for at least 4 hours to firm up before serving.