Add flour, salt, and butter to a food processor and pulse until a crumbly mixture forms. If you don’t have one, simply rub the butter into the dry ingredients.

Remove the dough and form a rectangular shape. Wrap in plastic wrap and chill for 30 minutes.

Remove the chilled dough and roll it out into a larger rectangle. Fold the dough like you would a letter you wanted to place inside an envelope.

Wrap it in plastic wrap and chill for an additional 30 minutes.

Remove the dough, roll it out, and fold it like a letter once again. This creates layers of a buttery pastry.

Cut 3 strips of parchment paper and place them in between the folds of the dough. Wrap it in plastic wrap and chill for at least 2 hours.

Add apple chunks, brown sugar, butter, cornstarch, cinnamon, and salt to a medium-sized saucepan over medium-low heat.

Cook, stirring frequently, until the butter is melted. Bring the mixture to a simmer for 5 minutes, stirring occasionally. The mixture should thicken.

Remove from the heat and stir in the vanilla extract.

Remove the sourdough puff pastry from the refrigerator and use a rolling pin to roll it into a large rectangle onto a lightly floured work surface.

Cut the dough into 4 1/2” squares using a pizza cutter.

Transfer each square piece of dough to a parchment-lined baking sheet.

Scoop about 1 1/2 Tablespoons of the cooled apple mixture to each square just a little off center of the middle of the dough.

Fold the squares over the filling and use a fork to crimp around the edges.

Brush the tops with egg wash using a pastry brush and use a sharp knife to cut slits in each turnover to vent.

Sprinkle with coarse sugar and bake at 400°F for 25 minutes until the pastry is golden brown.