Preheat the oven to 190ºC (375ºF), rack in the middle.
Line a 12 cup muffin pan with paper cups.
In a medium size mixing bowl mix together dry ingredients: flour, baking powder, salt. Set aside.
With electrical mixer beat softened butter and sugar at high speed, until light and fluffy.
Add the following ingredients one at a time and keep mixing until fully incorporated: sourdough starter, eggs, vanilla extract, lemon zest.
Incorporate flour mixture and milk into the batter by alternating 1/3 of flour mixture and 1/3 of milk and mixing it together using large silicone spatula.
Add frozen berries and gently fold into the batter by using silicone spatula.
Divide the batter among 12 cups in the pan, sprinkle the tops with a little bit of granulated sugar.
Bake for 35 minutes.
Let cool for 10 minutes in the pan and then remove the muffins and let completely cool on a cooling rack.