Salted-Butter Apple Galette with Maple Whipped Cream

Place a rack in middle of oven and preheat to 375 °. Place butter in a small saucepan and scrape in vanilla seeds; add pod. Cook over medium heat, stirring often, until butter foams, then browns (be careful not to burn), 5-8 minutes. Remove pan from heat and remove pod. Roll out dough on a lightly floured surface into a rough 14x10" rectangle about 1/8" thick (alternatively, roll out into a 12" round). Transfer to a parchment-lined baking sheet. Arrange apples on top, overlapping and leaving a 1 1/2" border. Brush apples with brown butter and sprinkle with muscovado sugar. Lift edges of dough over apples, tucking and overlapping as needed to keep rectangular shape. Beat egg with 1 teaspoon water in a small bowl and brush crust with egg wash. Sprinkle with granulated sugar and bake, rotating once, until apples are soft and juicy and crust is golden brown, 40-50 minutes. Let cool slightly on baking sheet before slicing. Beat cream in a medium bowl to medium-soft peaks. Fold in maple syrup and serve alongside galette. Do ahead: Galette can be baked 2 days ahead. Store tightly wrapped at room temperature. Reheat slightly before serving.