In a large pot, cook spinach in boiling salted water until wilded and bright green; then remove spinach with a slotted spoon and immediately plunge into the ice water bath to stop the cooking. Squeeze spinach with your hands to remove excess liquid.
Transfer spinach, milk, eggs and melted butter to a bender and puree.
Butter a large mixing bowl and set aside.
In a large bowl, whisk together flour, salt, and yeast. Make a well in center of mixture. Add spinach mixture and mix until dough forms a slightly sticky ball. Knead the dough on a floured surface until smooth and elastic, shape into a bowl and transfer to the buttered bowl. Brush top of the dough with more butter. Cover with plastic wrap and let stand in a warm place until it doubles in volume, about an hour.
Butter a 9 by 5 inch loaf pan. Punch dough down, shape into an 8.5 inch rectangle, fold long sides in to the middle, overlapping slightly. Press seam to seal and transfer dough, seam side down, to the loaf pan. Cover loosely with plastic and let stand until dough rises about an inch above top of pan, about 45 minutes.
In the meantime preheat oven to 390F and bake until top is golden brown for 25-30 minutes.
Let cool completely before slicing.