1. Combine 2/3 cup warm water and yeast in the bowl of a stand mixer fitted with dough hook, and let stand 5 minutes or until bubbles form. Weigh or lightly spoon 6.75 ounces bread flour (about 1 1/2 cups) into dry measuring cups; level with a knife. Sprinkle 6.75 ounces bread flour over yeast mixture. Add 1 tablespoon olive oil and salt. Mix at low speed for 2 minutes or until dough forms a ball. Increase speed to medium, and mix 8 minutes or until smooth and elastic. Place dough in a medium bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place, free from drafts, 1 hour or until doubled in size. (Gently press two fingers into dough. If indentation remains, dough has risen enough.) Punch dough down.
2. Cover surface of dough with plastic wrap lightly coated with cooking spray. Refrigerate for 24 hours. Remove the dough from refrigerator. Let stand, covered, for 2 hours or until dough comes to room temperature. Punch dough down. Place a piece of parchment paper on a flat work surface, and sprinkle parchment with remaining 2 tablespoons bread flour. Roll dough out to a 13-inch circle. Slide the parchment and dough onto a pizza peel or flat baking sheet. Brush the dough evenly with remaining 1 1/2 teaspoons olive oil. Cover dough lightly with plastic wrap.
3. Position an oven rack in the lowest setting, and place a pizza stone on rack. Preheat oven to 500°.
4. Combine milk and next 3 ingredients (through egg yolk) in a medium bowl, stirring with a whisk. Melt butter in a medium saucepan over low heat. Add garlic to pan; cook for 3 minutes or just until butter and garlic begin to brown, stirring frequently. Add 2 teaspoons all-purpose flour to pan, and cook 30 seconds, stirring constantly with a whisk. Stir in milk mixture, and bring to a boil. Cook for 1 minute or until thick, stirring constantly with a whisk. Spread sauce in an even layer over dough, leaving a 1/4-inch border; sprinkle sauce evenly with cheese. Slide parchment and pizza onto preheated pizza stone. Bake at 500° for 18 minutes or until crust is golden. Arrange prosciutto on pizza, and sprinkle with chives. Cut pizza into 12 wedges.