Preheat griddle to medium. Allow griddle to come to desired temperature before cooking french toast.
Combine egg yolks, heavy cream, and vanilla in shallow, flat bottomed dish. Soak each slice of bread on both sides in mixture.
Grease griddle with butter. If griddle is non-stick, butter may not be needed. Add soaked slices to hot griddle and allow to cook, flipping only once, until golden brown on each side.
To add caramelized sugar layer, add thin layer of sugar to the top of each piece of cooked French toast and caramelize until golden and bubbly with a cooking torch. Alternatively, you may arrange the slices in a single layer on a baking sheet and set under the broiler until golden brown.