Place a 12-inch cast iron skillet on the lower middle rack in the oven and heat the oven to 400°F.
Measure out 2 tablespoons of roast drippings and add to a food processor or blender, along with the flour, milk, eggs, and the salt. Process until smooth, about 30 seconds.
Carefully remove the hot skillet from the oven. Add an additional 2 tablespoons drippings to the skillet and swirl to coat the bottom of the pan. (If you don’t have 2 tablespoons for the skillet, add oil to make up the difference.) Pour the batter into the skillet, then return to the oven and bake until the pudding is puffed and golden brown, about 30 minutes. Serve hot alongside thick slices of standing rib roast.