Bring several quarts of water to a boil in a large Dutch oven. Add broccoli florets, and simmer 2 minutes or until crisp-tender. (Cook in batches, if necessary.) Drain and plunge broccoli into a bowl of ice water to stop cooking process. Drain and pat dry.
Melt 3 tablespoons butter in a large skillet over medium-high heat. Add half of pancetta and half of garlic; sauté 2 to 3 minutes or until lightly browned. Add half of broccoli florets, and sauté 2 to 3 minutes until thoroughly heated. Add 1 tablespoon lemon zest and 1/2 teaspoon salt; toss well. Transfer to a large bowl; cover and keep warm. Repeat procedure with remaining 3 tablespoons butter and remaining half of ingredients. Serve warm or at room temperature.
Make Ahead: Cook broccoli and pat dry a day ahead; store in zip-top plastic bags in refrigerator.
Note: Find pancetta in the deli meat department of large supermarkets. Regular bacon makes a fine substitute in this recipe.