On a stovetop, setup a bain marie or heat safe bowl over a pot of hot water partially filled to create steam. Add egg yolks to bowl and let the pot water boil underneath, stirring eggs with a whisk or fork constantly so the eggs don't scramble or cook.
When you see steam rising out from the pot, add the vinegar, tarragon, olive oil, and pepper. Keep stirring for several minutes until you get a thicker consistency. Remove from heat
If the sauce becomes too thick, simply stir in a little water. Or if too runny, add a little olive oil.