If you are going to bake the cookies straight after forming them, preheat the oven to 350F (180C). If you are going to ferment them, do not preheat the oven.
Cream the softened butter and sugar using a hand mixer or stand mixer. The mixture should be light and fluffy.
Now add the egg and egg yolk, vanilla extract and sourdough starter to the butter and sugar mixture and mix on moderate speed until well combined.
Next add the flour, baking powder and pinch of salt to the wet ingredients and bring together into a dough (you can use a wooden spoon or your hands if you prefer - I find this step easier to do by hand as the stiff dough will bog the hand mixer).
Use a teaspoon to scoop out balls of dough. Roll the balls a little and place them onto the prepared baking trays. I find the balls are perfect around 15g each, but you can go up to 20g.
Use a small spoon (half teaspoon is perfect) to make a small indent in the centre of each cookie. This is the "thumbprint" but a spoon is much easier.
** if you want to ferment the cookies, place the cookies into the fridge at this point. You can leave them up to 24 hours in the fridge)** (see notes for further information).
Use a small spoon to place a little jam into the indent in each cookie. You don't want to overfill them or they will make a mess in the oven.
Place the sourdough thumbprint cookies into an oven at 350F (180C) for around 15 to 18 minutes or until they are just golden and the jam is bubbling.
Remove from the oven and allow to sit for 5 minutes on the cookie sheet before transferring to a wire rack to firm up.