Preheat the oven to 375°F (190°C). Lightly grease an oval baking dish, approximately 11 × 7 inches (28 × 18 cm), with cooking spray.
If using frozen hash browns, thaw them completely. Transfer the potatoes to a clean kitchen towel and squeeze very well to remove as much excess moisture as possible.
Place the potatoes in a bowl and add the melted butter, extra virgin olive oil, salt and black pepper. Mix until evenly coated.
Spread the potatoes evenly over the bottom of the baking dish. Press down gently with the back of a spoon or a spatula.
Bake the hash brown base at 375°F (190°C) for about 25 minutes, until the edges begin to turn golden and the surface is lightly crisp.
Remove the dish from the oven and scatter the bacon evenly over the top.
Return the dish to the oven and bake for another 5 minutes.
Meanwhile, whisk the four eggs with the whole milk, onion powder and paprika until smooth and well combined.
Remove the dish from the oven and slowly pour the egg mixture evenly over the hash brown and bacon base.
Reduce the oven temperature to 350°F (180°C).
Cover the dish with aluminum foil and bake for about 18 minutes.
Remove the foil and bake for another 15 minutes, or until the egg mixture is fully set in the center and the potato edges are golden. The center should reach 160°F (71°C).
Remove the casserole from the oven and let it rest for about 10 minutes before slicing.
While the casserole rests, prepare the hollandaise sauce.
Place the two egg yolks, heavy cream, lemon juice, Dijon mustard, cayenne and salt in a blender.
Blend for about 10 seconds, until combined.
With the blender running, slowly pour in the melted unsalted butter in a thin stream.
Continue blending until smooth and creamy, thick enough to coat a spoon but still pourable.
If needed, keep the sauce gently warm until serving, without allowing it to boil.
Cut the casserole into four portions.
Top each portion with hollandaise sauce and freshly chopped chives.
Serve immediately.