In a medium saucepan, melt butter over medium heat. Add apple, brown sugar, lemon juice, vanilla, cinnamon, and salt. Bring to a boil over medium-high heat, stirring occasionally. In a small bowl, whisk together 2 tablespoons (30 grams) water and cornstarch until smooth; stir into apple mixture. Cook, stirring constantly, until mixture is thickened and translucent and apples are tender, about 3 minutes. Remove from heat, and let cool completely.