In a heavy pan mix the egg yolks and sugar together.

After mixing the egg yolks and sugar together, add the lemon juice and mix together.  Heat over medium heat until mixture thickens to where it coats the back of the spoon.

Do not let the mixture boil or really even almost boil or you will have curdling problems too.

Once the mixture thickens, remove from the heat. Add the butter and stir until melted.  If curdling occurred (i.e. there are little bits of scrambled egg in your curd) strain through a fine mesh strainer.

After straining, mix in the extracts.  If you prefer, you can use the zest from one lemon instead of the lemon extract. The high amount of lemon juice in this curd recipe makes it very lemony with or without extract.