Buttermilk and berries combine well to fill tarts with sweet creaminess. Using a blender or food processor, thoroughly blend together 1 cup buttermilk, 3 large eggs yolks, 1/2 cup sugar, 1/4 cup cooled melted unsalted butter, 1 tsp. vanilla, 1/2 tsp. salt, 2 tbsp. all-purpose flour, and 1 tbsp. each of lemon zest and lemon juice. Line prebaked tart crust with the freshest berries available and pour the mixture over the top. Leave 1/8-inch space at the top of crust to prevent spills. Bake in a preheated 350 degree F oven for around 30 minutes or until the filling is firm.