In a mixing bowl, combine flour, sugar, and salt.
Add chilled butter and mix until crumbly.
Stir in sourdough starter, milk, and vanilla until a dough forms.
Wrap the dough in plastic wrap and refrigerate for 30 minutes.
Preheat the oven to 375°F (190°C).
Roll out the dough on a floured surface to about 1/8 inch thick.
Cut the dough into rectangles, about 3x4 inches.
Place a tablespoon of your chosen filling on half of the rectangles.
Top with another rectangle and seal the edges with a fork.
Bake for 20-25 minutes until golden brown.