Combine 1/2 pound huckleberries with 1/4 cup of sugar in a small saucepan over gentle heat. Stir just until the berries release their juices. Press through a fine strainer, throw away the solids, reserve juice, and let cool. Cream 5 tablespoons room-temperature butter in a medium stainless steel bowl. Add 1/4 cup sugar, beat until light. Add 2 room-temperature egg yolks, then 2 room-temperature large eggs, beating well after each addition. Stir in 1/8 teaspoon fine salt, 1/2 cup reserved juice, and 1 1/2 teaspoon late harvest apple vinegar.
Fill a small saucepan 1/3 full with water. Bring to a simmer, and place the bowl of curd on top of it. Stir constantly, heating the curd slowly, working to 166F over ten minutes. Pull the curd from the heat when it is thick enough to coat your spoon, it will thicken substantially as it cools. Keep refrigerated for up to a week, or up to a month in the freezer.
Makes about 1 1/2 cups.