Bring approx. 3 litres of water to the boil in a saucepan. Add vinegar and salt. Beat the eggs one after the other into a cup. Make a whisk in the vinegar water and let the eggs slide into the whisk one by one. Cook for about 4 minutes. Lift them out and let them drip off on kitchen paper.
Halve the avocado, remove the core, remove the flesh from the skin and cut into slices. Sprinkle with 1 tablespoon lemon juice. Cut ham slices in half. Wash chives, shake dry and cut into fine rolls.
Melt the butter. For the sauce, mix the egg yolks, yoghurt, 1 pinch of salt and 1 tbsp. lemon juice in a tall, narrow beaker with the blender until smooth. Slowly pour the butter into the egg yolk mixture. Continue to puree until a smooth sauce is obtained.
Halve the toasties and toast them. Cover toast halves with ham, avocado and eggs, sprinkle with chives and drizzle with the sauce. Sprinkle with coarse pepper.