Line a rimmed baking sheet with parchment paper.

In a large bowl, stir together apples, sugars, pie spice, and salt. Let stand for 30 minutes, stirring occasionally. Stir in cornstarch.

In a medium saucepan, melt butter over medium-high heat. Add apple mixture and ⅓ cup (80 grams) water, and bring to a boil. Reduce heat to medium; cover and cook, stirring frequently, until apples are just softened, about 10 minutes. Uncover and cook, stirring frequently, until liquid has thickened and reduced and apples are very soft, 5 to 10 minutes. Remove from heat; stir in vanilla.

Transfer apple mixture to prepared pan, spreading into a thin layer. Let cool completely, 30 to 40 minutes.