Whip the softened butter with vanilla. Then slowly add in 2-3 cups of the powdered sugar. Add the strawberry jam and whip. Add the rest of the sugar and mix well. If it's too thick, add a little milk (very little, 1 tsp at a time) until you reach your desired consistency. It should be fairly thick so it won't spread once you pipe it on the cupcake.
Recipe by Honeybear Lane at https://www.honeybearlane.com/2013/04/strawberry-jam-frosting-recipe.html