Method:

Pour vinegar into a small pan with the peppercorns and bay leaf.
Reduce the vinegar over a high heat, until there is only 1 tbsp left.
Strain out the peppercorns and the bay leaf and let it cool.
Once cooled, pour the vinegar reduction into a large, heat-proof bowl and whisk in two fresh egg yolks.
Place the bowl over a saucepan of simmering water, without letting it touch the water, and whisk until lukewarm.
Melt the butter and slowly add it to the egg yolks whilst beating continuously. Once the mixture has thickened, remove from the heat.
Season to taste with salt, pepper and lemon juice.