Preheat the oven to 350 degrees F and grease a 12-hole muffin pan with coconut oil (or butter/cooking oil of choice).

In a mixing bowl, whisk together the sourdough starter, egg, coconut oil and vanilla extract. Continue whisking until very well-combined and there is no more oil separation.

In a separate bowl, combine the flour, sugar, baking soda, and sea salt.

Add the dry mixture to the mixing bowl with the wet mixture and stir until well combined. The batter will be very thick. Add the blueberries and gently fold them in (so as to avoid much breakage).

Fill the muffin holes ⅔ to ¾ of the way full with batter (batter will rise).

In a small bowl or measuring cup, combine the ingredients for the topping and stir well. Use your hands or a spoon to sprinkle topping generously over the blueberry muffin batter.

Allow muffins to cool at least 20 minutes. Gently run a butter knife between the muffins and the muffin tray to pop them out.