In a medium skillet over medium-high heat, melt half of the butter. Add the Canadian bacon and cook until golden on both sides, about 6 minutes total, flipping halfway through. Set aside.
Fill a large, nonstick skillet with 1 inch of water. Add the salt and bring to a simmer over medium heat. Crack 1 egg at a time into a small ramekin, then gently pour it into the simmering water without allowing the eggs to touch. Remove the pan from the heat, cover, and let the eggs poach undisturbed for 5 minutes. Using a slotted spoon, transfer the eggs to a shallow bowl and reserve.
Toast the bread slices and butter, using the remaining butter.
Make the hollandaise sauce: combine the egg yolks, lemon juice, and salt in a blender, pulse a few times to combine, then, with the blender running, slowly add the melted butter and blend until smooth. Blend in the hot water, then season to taste with cayenne pepper and salt.
To serve, divide the toasted, buttered bread between 4 plates. Top each bread slice with a tomato slice, a slice of Canadian bacon, and a poached egg. Pour some of the hollandaise sauce over the egg, garnish with paprika and chives, and serve immediately.