In a stand mixer, combine sourdough discard, milk, flour and sugar until a stiff batter forms. Mix well. Cover and place in fridge to ferment overnight if desired.
Add remaining muffin ingredients into the stand mixer, reserving the cranberries. Mix until everything is combined and there are no lumps, being careful not to overmix or you'll end up with dry, dense muffins.
Quarter your fresh cranberries and gently fold into the batter. If using dried cranberries, let the cranberries soak in orange juice for 1-2 hours (or overnight) before mixing up your muffin batter.
Line your muffin tins with muffin liners and fill each cup nearly to the top.
Prepare the Crumble
Melt butter in a microwave safe dish and add flour, sugar and a sprinkle of sea salt. Mix well with a fork until you reach your desired texture.
Place a large spoonful of crumble on top of each muffin.
Bake your muffins
Preheat your oven to 425 °F
Bake your muffins at 425 °F for 12 minutes. Then turn the heat down to 350 °F for another 8-10 minutes. You'll know your muffins are done baking when you stick a toothpick in the center of the largest muffin and it comes out clean.
Cool & Glaze
Let muffins cool for 10-15 minutes on a wire cooling rack.
Make your glaze by whisking together powdered sugar and orange juice. Start with just one tablespoon of orange juice and then slowly add more until you reach your desired drizzling consistency. Drizzle over muffins and enjoy!
Notes
These muffins are best day of, but will stay delicious on the counter for about 3 days. If you don't think you'll eat them all faster than that, I recommend freezing your muffins after they've cooled so they stay fresh. Freeze up to 3 months. If you don't have fresh cranberries, you can substitute dried cranberries. To prevent a really chewy texture, plump the dried cranberries in orange juice while your batter does it's first ferment - at least an hour or overnight. Then pour in the orange juice and dried cranberries when you make your batter.