Concord Grape Curd

1 1/2 cups concord grapes (slightly smashed) + 1/2 cup sugar + 1 tsp lemon zest and 1/3 cup lemon juice + 4 egg yolks + hefty pinch of coarse salt + 1 stick unsalted butter cut into small cubes. whisk over medium-low heat until thick and mixture just starts to bubble/boil. strain + chill.