In a large bowl place the whole wheat flour and the white flour. Add the cubed butter and mix it in with your fingertips or with a pastry cutter until it resembles coarse crumbs.
Mix in the sourdough starter.
Add the rest of the ingredients to the bowl except the baking soda. Mix until well combined.
Cover the batter with a plastic wrap or a damp tea towel and leave somewhere warm for 6-8 hours or overnight. The dough will likely puff a little but it won't rise a lot like bread dough would.
After your batter has been fermented, preheat your oven to 180C/350F. Sprinkle baking soda on top of your batter and mix it in well with a wooden spoon. Grease or line your muffin cups and fill about 2/3 full with the batter.
Bake for about 15 minutes until risen and golden brown on top.
Let your muffins cool on a cookie rack. They are delicious served as a snack with some kefir or sour milk.
Notes
You can use sourdough discard instead of active starter in this recipe and skip the fermenting step if you wish.
These muffins freezer really well and are quick to defrost.