Preheat the oven to 400° F.
Prepare your muffin tins. You can either line them with parchment paper, paper muffin liners, or grease your muffin tin with coconut oil.
Add all of the dry ingredients, all-purpose flour, sugar, salt, baking powder, and baking soda, into a large bowl and whisk until combined.
In a small sauce pan, melt one stick of butter.
In a medium bowl, add the sourdough discard, milk, eggs, vanilla, lemon juice, and lemon zest to a bowl. Combine the wet ingredients together and slowly add the melted butter to the mixture, while continually whisking. Continuing to whisk is super important in this step so that the hot butter does not cook the eggs or curdle the milk.
Once everything is combined, add the wet ingredients into the dry ingredients and gently mix using a rubber spatula until everything is just combined. Do not over-mix.
Next, gently fold in 2 cups of fresh or frozen blackberries.
Using an ice cream scoop or dry measuring cup, scoop the lemon blackberry batter into a lined or greased muffin tin.
Bake at 400° F. for 20-25 minutes or until they are golden brown and a toothpick comes out clean.
Allow the muffins to fully cool on a wire cooling rack before eating. This will allow the muffins time to rehydrate and cool down, creating a lighter, fluffier texture.