Roll out 6 feet of cheesecloth and cut in half.
Place roast at one end of first piece and roll up.
Rotate roast 90 degrees and roll in second piece of cheesecloth.
Set wrapped roast on cooling rack inside quarter sheet pan; tie roast to rack standing on its side.
Refrigerate for 4 to 5 days.
Remove roast from fridge; unwrap and discard cheesecloth.
Rub roast with 2 tablespoons beef fat or tallow.
Season with 2 teaspoons salt per bone and sprinkle with pepper.
Leave uncovered at room temperature for 1 hour.
Prepare probe thermometer; insert as directed above fat ridge.
Set thermometer alarm to 118°F.
Place roast on lower middle rack in cold oven.
Set oven to 250°F and roast until meat reaches 118°F, about 3 to 4 hours.
Remove roast, cover with foil, let rest while preparing pudding.
Place empty skillet on lower middle rack; increase oven to 400°F.
After 10 minutes rest, transfer roast to cutting board.
Pour drippings from sheet pan into heatproof liquid measuring cup.
Return foil-covered roast to pan.
Measure 2 tablespoons drippings; add to blender with eggs, 1 1/2 teaspoons salt, milk, and flour.
Blend until smooth, about 30 seconds.
Remove hot skillet; add additional 2 tablespoons drippings, swirl to coat bottom.
Pour batter into skillet; bake until pudding is puffed and golden, about 30 minutes.
Transfer pudding to rack and loosely cover with foil.
Increase oven to highest setting.
Return roast to oven to deeply brown exterior, about 10 minutes.
Carve roast and serve alongside pudding.