Melt the butter and set aside
Using a double boiler, bring the water in the lower level to a simmer.
In the top pot, add the yolks, lemon juice or vinegar and whisk until sauce starts to thicken.
Slowly add the melted butter in a slow constant stream, while continuing to whisk until the sauce thickens.
Remove sauce from heat and cover to keep warm.
Toast the muffins, arrange the ham and 2 kale leaves on top and set aside.