In a small bowl, whisk together 3 T. melted butter, 6 T. brown sugar, 2 T. of the apple butter, egg yolk & 1/2 t. vanilla extract. Whisk very well to blend.
Spread the batter in the loaf pan, & dollop the remaining tablespoon of apple butter over the surface. Use a knife to swirl in the apple butter. Bake for 19-21 minutes at 350 F.