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.
In a large bowl, whisk together flour, salt, and yeast. Make a well in center of mixture. Add spinach mixture to well stir until a drought forms. Transfer to a lightly floured work surface and knead until dough is smooth and elastic. Transfer to a bowl brushed with butter; brush top of dough with more butter. Cover bowl with plastic wrap and let rise until doubled in volume, 45 minutes to 1 hour.
Butter a 9 by 5 inch loaf pan with butter . Place dough, top-side down, on lightly floured work surface to deflate. Pat dough into a 9-inch square using your hands. Fold bottom edge two thirds of the way up, and fold again so that the top edge meets the bottom edge as you would a business letter. Pat dough a second time into a 9-inch square. Roll dough to form a tight log and pinch to seal. Pat the ends just enough to fit into the pan, and place the dough seam side down into the prepared pan. Cover with plastic wrap sprayed with cooking spray. Let stand in a warm place for 45 to 60 minutes.
Preheat oven to 390 degrees with rack set in the center. Remove plastic wrap and transfer to oven and bake for 25 minutes.
Transfer to a wire rack to cool slightly; turn out loaf. Brush top with butter to coat and let cool almost completely before slicing.