Combine sourdough starter, butter, fine sea salt and rye flour in a medium bowl.  Mix and knead together with your hands until a cohesive dough forms.  Wrap dough tightly in plastic wrap and place in the fridge until well chilled, 2 hours or up to 1 day.

Preheat the oven to 350 degrees.  Remove dough from the refrigerator and tear off a piece that is about 2 inches in diameter.  Using your hands, roll the piece of dough into a ball and place on a piece of parchment paper.  Flaten slightly into a disk shape with your palm.

Sprinkle dough round with flour and roll with a traditional rolling pin until it is VERY thin circle, about 8 inches in diameter.  Sprinkle with additional flour as necessary to prevent sticking.

Switch to the kruskavel.  Roll back and forth creating dimples in the surface of the crispbread.  Alternatively, you can use a fork to prick holes in the dough, but it won’t have the same appearance as crispbread that has been rolled with a kruskavel.

Using a 1 ½ inch biscuit cutter, cut a hole in the center of the dough round, leaving the circle in place while the crispbread bakes.

Sprinkle with flaky sea salt.  Transfer parchment paper to a rimmed baking sheet and bake for 10-16 minutes or until crispbread is golden brown and crisp.  Do not remove it from the oven too early!  No one likes soggy crispbread.

Transfer to a wire rack to cool.  Repeat with remaining dough.  Store in an airtight container at room temperature for up to two weeks.