Melt your butter in a small pan on a medium-low heat (or in a microwave). Take it off the heat as soon as it melts and pour into a jug. Pop your egg yolks in a small bowl, then using a hand whisk or a hand blender (which is better as your bowl will move around less) keep mixing as you very slowly drip in your butter. Keep it slow as this is when it can curdle. Once you’ve added half your butter you can speed up to a steady, slow pour. Mix in your lemon juice and season with salt and pepper. Done.