To make mock hollandaise, combine buttermilk, mayonnaise, lemon juice, and yellow mustard in a small saucepan over low heat, stirring well with a whisk. Add butter and stir until butter melts. Keep warm.
In a large shallow saucepan, add water and vinegar. Bring to a boil, and reduce heat to a steady simmer.
Break each egg into a custard cup, and gently pour into the pan.
Simmer the eggs until the whites are firmly set, about 3 minutes.
While eggs are poaching, cook the Canadian bacon in a medium skillet until nicely browned.
Remove each egg with a slotted spoon and place gently onto a towel to drain.
Place one muffin half on a plate and top with a slice of Canadian bacon and one poached egg.
Top off with 2-3 Tablespoons of the Hollandaise sauce.
Garnish with chopped chives, serve and enjoy your Eggs Benedict!