Blueberry Muffins with Banana Butter

Name: Blueberry Muffins with Banana Butter
Ingredients: 2 c. all-purpose flour; 2 tsp. baking powder; ¼ tsp. kosher salt; 1 stick unsalted butter; ½ c. whole milk; ¾ c. granulated sugar; ¼ c. light brown sugar; 1 tsp. finely grated lemon zest; 2 large eggs; 1½ c. blueberries; 6 tbsp. unsalted butter; 1 very ripe banana; ½ tsp. freshly squeezed lemon juice; 1 kosher salt