1. Strain beaten egg yolks through a sieve into a heavy saucepan.
2. Add honey and lemon juice and stir to combine.
3. Cook over low or med/low heat stirring constantly with a wooden spoon until mixture is thick and coats the back of the spoon.
4. Remove from heat, stir till mixture cools a bit and then stir in the butter pieces one at a time. Keep stirring all the while till all butter is mixed in.
5. Stir in the grated zest and cool completely. Refrigerate or if using in a tart, pour into tart shell and then put in the fridge.