Preheat the oven to 375 degrees. Lightly grease a standard12-cup muffin pan. In a skillet, cook the bacon over medium heat, turning occasionally, until crisp, about 5 minutes. Transfer to paper towels to drain. Let cool slightly, then coarsely chop.
Pour off all but 2 tbsp. of the bacon fat from the skillet; add 4 tbsp. butter, the onion and celery. Cook, stirring often, until the vegetables are tender, about 5 minutes. Let cool.
In a bowl, whisk together the flour, cornmeal, parsley, baking powder, sage, salt and pepper. Form a well in the center, then add the buttermilk, eggs, onion mixture and bacon. Stir just until batter comes together.
Using an ice cream scoop, fill the muffin cups to the top with the batter. Bake until the muffins are springy, about 35 minutes. Let cool in the pan for 10 minutes, then slide a knife around the rim to remove. Serve warm with butter. Serve warm with butter. [Make ahead: Store the muffins in foil or resealable plastic bags and refrigerate for up to 2 days or freeze for up to 1 week. Reheat in a clean muffin pan in a 350 degree oven for about 20 minutes.]