In a small pan over medium low heat* combine the yolk, a good squeeze of lemon juice and half of the butter. Stir constantly until thick (about 2 minutes). Add the other half of the butter and stir until thick. Serve immediately.
If your worried about the eggs scrambling you can use a double boiler or hold the pan over boiling water and whisk the sauce until thick. The recipe works with less butter but the more butter you use the richer the sauce will be:)