Preheat the oven to 400F (200°C).

In a small bowl, weigh out the called for blueberries. In another small bowl, transfer about 50g of the blueberries and mash with a fork.

In a medium mixing bowl, measure out the called for flour. Remove about a tablespoon of this flour and add it to the bowl with whole blueberries, tossing the blueberries to evenly coat with the flour.

Add the salt, baking soda, and baking powder to the flour and whisk to combine.

In a large mixing bowl, add the melted and slightly cooled butter with the sugar. Stir to dissolve.

Add eggs to the butter and sugar, one at a time, mixing after each until incorporated. Add vanilla.

Measure out the sour cream into a measuring cup and combine it with the sourdough starter. Stir until smooth. Add this mixture to the butter/sugar and mix to combine.

Add the zest of one lemon (optional), the mashed blueberries, and the sourdough starter mixture to the butter/sugar and mix to combine.

Add the flour mixture to the butter/sugar, mixing just until no dry bits of flour remain. Fold in the floured blueberries.

Line a 12-cup muffin tin with muffin liners, and fill each well to the rim. Sprinkle coarse sugar over each well (optional).

Bake at 400°F (200°C) for 10 minutes then reduce the oven to 350°F (175°C) and bake for 25-30 minutes longer until the tops are slightly golden.

Remove muffins from the oven and cool for 5 minutes in the pan. Then remove them to a wire rack. Store loosely covered on the counter.