Preheat the oven to 400°.
Line a muffin pan with paper liners and set aside.
In a large bowl, mix together the flour, baking powder, baking soda, salt and lemon zest until combined.
In a medium bowl, whisk together the egg and the sugar until well blended and pale yellow.
Pour in the melted butter and whisk to combine. Lastly, add the buttermilk and vanilla to the sugar mixture, whisk until fully incorporated. The mixture will be very thin.
Add the wet ingredients into the dry ingredients and gently mix until almost fully combined. The batter should be lumpy with flour streaks showing still.
Add the frozen berries to the lumpy batter.
Quickly and gently, fold the berries into the batter until the berries are evenly distributed and no flour streaks remain.
Divide the batter between the 12 muffin cups and bake at 400° for 20-23 minutes. The muffins are done when a toothpick inserted comes out clean.
Allow to cool just long enough to handle, then spread on some salty butter and enjoy!