Preheat oven to 375° and line two baking sheets with parchment.
In a large bowl, whisk together the flour, baking soda, and salt. Add the cold butter, working it into the dry ingredients with your fingers until it resembles coarse meal.
Slowly add the milk and gently mix the dough, then knead it, until it just comes together and is tacky but not sticky. (If necessary, add more flour.) Wrap in plastic and refrigerate for an hour.
Turn the dough out onto a floured surface. Divide in half, and return one half to the refrigerator. Roll out the other half ⅛ inch thick, sprinkling with more flour as needed to prevent sticking. Use a pizza cutter to cut it into 2-inch squares.
Transfer the squares to the baking sheets, leaving an inch of space between squares; then sprinkle with coarse sea salt, and prick them all over with a fork.
Bake until golden brown and crisp, about 15 minutes, depending on size.
Repeat with the second half of the dough.