Preheat your oven to 375°F / 190°C. Lightly grease a baking sheet with butter or a neutral oil.
If you're not using store-bought buttermilk, make your own now. Gently warm the milk (about 1 minute in the microwave is fine) and stir in the vinegar or lemon juice.
Set it aside for 5-10 minutes to curdle while you prepare the other .
In a large bowl or the basin of a food processor, whisk together the flour(s), salt, baking soda, and baking powder until well combined.
Pour in the buttermilk (or your soured milk mixture) and mix until a soft, shaggy dough forms. Be careful not to overmix. If using a food processor, pulse for about 30 seconds. If by hand, knead gently on a floured surface for just 2-3 minutes until it comes together into a smooth ball.
Shape the dough into a round loaf and place it on your prepared baking sheet.
Using a sharp knife or razor blade, score a deep 'X' across the top, about half an inch deep.
Bake for 45-55 minutes, or until the loaf is a deep golden brown and sounds hollow when you tap the bottom. An instant-read thermometer inserted into the center should read about 210°F / 99°C.
Transfer the loaf to a wire rack and let it cool completely before slicing. This is important for the texture!
Don't overwork the dough.
Mix only until the are just combined. Over-kneading will develop the gluten too much and result in a tough, chewy loaf instead of a tender one.
Let it cool completely before slicing. Slicing into hot soda bread can cause it to be gummy and dense inside.
Patience will be rewarded with the perfect crumb structure.