Melt butter in a saucepan or in a microwave and allow to cool
Using the immersion blender, blend yolks for a couple of moments, then continue blending while drizzling in melted butter VERY slowly in a steady stream until fully incorporated.
Add cayenne, lemon juice and salt to taste- then blend one more time Use immediately or place in the fridge to store for a maximum of 2-3 days
As it will firm in the fridge, make sure to carefully reheat for the best texture.