1. Make the sauce Beat the egg yolks, lemon juice, buttermilk and pepper together in a microwave-safe glass mixing bowl until thick and light in colour (preferably with an electric beater). Gradually add the butter until the sauce is thick and smooth.
2. Now cook the sauce in the microwave on high for 1 minute, but beat every 15 seconds. Finally, gradually add the boiling water to dilute the sauce slightly and season with salt and fresh herbs to taste.
3. Cut the roosterkoek in half and toast in a dry pan. Place rocket, biltong and tomato on each bottom half, followed by an egg. Drizzle generously with hollandaise sauce and serve immediately!