Preheat oven to 400 degrees F (200 degrees C). Generously spray 12 muffin cups with cooking spray.
Melt butter in a skillet over medium heat; add bacon. Cook gently until bacon begins to curl, 3 to 5 minutes. Add onion, garlic, and chard leaves; cook and stir until chard has wilted, stirring occasionally, 5 to 8 minutes.
Transfer chard mixture to a large bowl; stir in zucchini, eggs, salt, and black pepper. Add flour and baking powder; mix until flour is evenly distributed and moistened. Stir in Swiss cheese. Spoon batter into prepared muffin tin.
Bake in the preheated oven until edges of muffin turn golden brown, about 20 minutes.
