In a mixing bowl, whisk together the flour, sugar, salt, and baking soda.
3 (375g) cups all-purpose flour, 3 (45g) Tbspns granulated white sugar, 1½ teaspoons fine sea salt, 1 teaspoons baking soda
Using a box grater, grate your cold butter into small bits. You can also cut the butter into small 1/4 inch cubes and use a fork or pastry cutter to cut the butter into the dough.
1/4 (57g) cup cold unsalted butter
Mix the butter bits into the dry ingredients. The goal is for their to be small pea-sized bits of butter throughout the dry ingredients.
Create a little dent or well in the center of the dry ingredients, and pour in your cold sourdough discard and the cold buttermilk.
1/2 (120g) cup sourdough discard, 1 (260g) cup buttermilk
Mix the dough gently with your hands or a dough spatula until a shaggy dough forms. The dough should feel soft and slightly tacky. If the dough is very dry add 1 Tablespoon of buttermilk at a time till the dough just comes together. If the dough feels too wet, dust it with flour and gently mix to absorb a bit of the moisture. Go slow adding anything extra to the dough, we want just enough to achieve the right consistency.
Bring the dough together until it forms a cohesive ball. The dough ball should be roughly 6-7 inches wide and about 2 inches tall.
Note: Soda bread should not be kneaded like yeasted bread, or it will make the dough tough.
Optional Cold Fermentation: If you want to give this dough a cold ferment, wrap the ball of dough tightly in plastic (you don't want any air reaching the dough). Air will work with the soda and create a greyish looking dough. Transfer the tightly wrapped dough to the refrigerator and allow it to rest overnight or until you're ready to bake the soda bread the next day.