Preheat oven to 350 degrees F. Line a 6-cup jumbo muffin pan with paper liners. 

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In a medium microwave-safe bowl, melt 4 tablespoons of the butter in the microwave and let it cool slightly. 

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In a bowl, stir together the flour, salt, baking powder, and baking soda. Add the lemon zest, sugar, buttermilk, milk, egg, honey, and vanilla to the slightly cooled butter and whisk until combined. Using a rubber spatula, fold in half of the flour mixture. Do not overmix. 

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Add the blueberries to the remaining flour mixture and toss to coat. Fold the blueberries and flour mixture into the muffin batter. 

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Divide the batter evenly among the paper liners. Top each with the coarse sugar sprinkles, if using. 

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Bake the muffins for 18 to 22 minutes, until lightly browned. Let them cool for about 15 minutes. Cut the muffins in half. 

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Grease a cast-iron griddle with the remaining 1 tablespoon butter and heat over medium-high heat. Working in batches, place the muffing halves on the griddle and cook until toasted and browned, about 30 seconds to 1 minute. Repeat with the remaining muffins. 

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Serve immediately with butter and a sprinkling of powdered sugar, if using. 

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