Preheat oven to 375 and fill 12 muffin cups with paper liners.
In a large bowl whisk the flour with baking powder and salt.
In a medium bowl whisk butter with the milk, both sugars, lemon zest and eggs.
Add the wet ingredients to the dry and whisk until just combined.
Fold in blueberries. Spoon batter into muffin cups and bake 20-25 minutes. Unmold and let cool on rack.
Make the banana butter. In food processor puree the butter, banana, lemon juice and salt.
Muffins can be stored in an airtight container for up to 2 days. The butter can be refrigerated for up to 1 week.
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