In a large bowl, combine 1/3 cup bread crumbs, green onions, egg, mayonnaise, lemon juice, garlic powder and cayenne; fold in crab.
Place remaining bread crumbs in a shallow bowl. Divide mixture into eight portions; shape into 2-in. balls. Gently coat in bread crumbs and shape into 1/2-in.-thick patties.
In a large nonstick skillet, heat butter over medium-high heat. Add crab cakes; cook until golden brown, 3-4 minutes on each side.
Add egg yolks, lemon juice, and salt to a tall, narrow cup that fits the head of an immersion blender.
Melt butter in a saucepan (or in the microwave!) until a thermometer registers the butter temperature at at least 200 degrees F.
Blend the egg mixture until egg yolks have slightly lightened in color, about 1 minute.
Stream hot butter into eggs a little at a time while you whisk, letting the hot butter cook the eggs and blending until hollandaise is light and fluffy and all butter has been incorporated. If the hollandaise gets too thick, blend in a few
teaspoons of water to loosen it before serving.
Poach each egg in hot water (water should be just-barely-simmering for best results) until whites have cooked through, about 4 minutes.
While eggs are poaching, toast your English muffin halves. Place each toasted half of English muffin cut-side up on a plate and top each half with a crabcake.
When eggs are done, gently place one egg on top of each muffin/crabcake stack and season eggs with a bit of pepper.
Pour hollandaise over the English muffin stacks. Top with a few fresh arugula leaves and serve immediately.