Preheat the oven to 375° and fill 12 muffin cups with paper liners.
In a large bowl, whisk the flour with the baking powder and salt. In a medium bowl, whisk the butter with the milk, both sugars, lemon zest and eggs. Add the wet ingredients to the dry ingredients and whisk just until combined. Fold in the blueberries.
Spoon the batter into the muffin cups and bake for 20 to 25 minutes, until the muffins are golden. Let cool for 5 minutes. Unmold the muffins and let cool on a rack.
unsalted butter, at room temperature
very ripe banana, sliced
fresh lemon juice
pinch kosher salt
In a food processor, puree the butter, banana, lemon juice and salt. Serve with the muffins.
The muffins can be stored in an airtight container for up to 2 days. The butter can be refrigerated for up to 1 week.