Melt the butter in a small saucepan or in the microwave. Pour the melted butter into a small pitcher with a spout.
Place the egg yolks and a splash of lemon juice in a tall cup large enough to fit your immersion blender. Insert the blender and turn it on.
Slowly pour the melted butter in a thin stream while keeping the hand blender running at a constant speed. The sauce will emulsify; continue blending until all the butter is incorporated and you have a smooth, homogeneous sauce.
Season to taste with parsley, chives, salt, and pepper. Serve immediately.