In a large bowl, combine the rye flour, sourdough discard, kosher salt, and milk. Mix with a fork or spatula until shaggy, then knead briefly by hand until the dough comes together into a stiff, slightly sticky ball.

Cover the bowl and rest at room temperature for 1–2 hours for same-day crackers, or leave at room temperature for 24–48 hours for a deeper, tangier flavor.

When ready to bake, preheat the oven to 350°F. Divide the dough into 6 equal pieces and keep them covered with plastic wrap so they don't dry out.

Working one piece at a time on a sheet of parchment, roll the dough into a thin rectangle, about 7–8 inches across. If it sticks to the rolling pin, lay a second sheet of parchment on top and continue rolling. Roll as thin as you possibly can.

Remove the top parchment. Cut the piece in half, separate the two pieces on the parchment, replace the top sheet, and continue rolling each piece thinner. Trim ragged edges with a pizza wheel if you want clean lines.

Poke the dough all over with a fork, then cut into your preferred size. Pieces around 1 inch by 4 inches work well; avoid pieces larger than 3 inches by 4 inches or the center may stay chewy.

Arrange the cut pieces slightly separated on a parchment-lined baking sheet. Brush lightly with softened butter and sprinkle with flaky sea salt.

Bake 15–17 minutes, until golden brown and crisp all the way through. Do not pull them early.

Transfer to a wire rack and cool completely before storing. Repeat with remaining dough pieces, baking as many trays at once as your oven allows.