Whisk flours, oats, salt, sugar and yeast together in a large bowl. Stir in milk and butter. Knead briefly in the bowl with your hands until the dough comes together (dough will be quite stiff and a bit sticky). Cover and let rise at room temperature for 1 hour.
Preheat oven to 375 degrees. Divide dough into 6 equal pieces and cover with plastic wrap. Working with one piece of dough at a time, roll into a thin round on a sheet of parchment paper using a traditional rolling pin.
Roll with a kruskavel a few times or poke all over with a skewer or fork. Cut a 1 inch hole out of the center using a biscuit cutter. Set the round cut out piece of dough next to the larger crispbread. Brush both with butter and sprinkle with sea salt crystals.
Transfer parchment paper to a baking sheet. Bake until golden brown and crispy, about 12-15 minutes. Transfer to a wire rack to cool. Repeat with remaining pieces of dough.
Roll your crispbread thin. PAPER thin. When you think it's thin enough, keep rolling. You will get a much more crisp final product when you take the time to roll it out as thin as you possibly can. You can also expect your crispbread to expand slightly when baked so it will not be quite as thin as when it went into the oven.
Bake until golden brown and crisp. DO NOT remove your knäckebröd from the oven too soon. This leads to soggy crispbread. You want to make sure it is crispy all the way through.
Allow the crispbread to cool completely on a wire rack prior to storing. Moisture is the enemy of good crispbread. Make sure that it is thoroughly cooled before placing in an airtight container.
Store the crispbread in an airtight container at room temperature for up to two weeks.
Calories: 228kcal | Carbohydrates: 37g | Protein: 8g | Fat: 6g | Saturated Fat: 3g | Polyunsaturated Fat: 1g | Monounsaturated Fat: 2g | Trans Fat: 0.2g | Cholesterol: 14mg | Sodium: 209mg | Potassium: 195mg | Fiber: 4g | Sugar: 2g | Vitamin A: 166IU | Vitamin C: 0.01mg | Calcium: 51mg | Iron: 2mg