In the mixing bowl, using your hand or stand mixer, cream together the sugar and butter until soft and fluffy, about two to three minutes.
Add in the eggs and mix until well combined.
Mix in active sourdough starter and vanilla extract.
Add in half of the flour, the salt, and the baking soda.
Add in the rest of the flour, mix until everything is well combined.
Chill dough in the fridge for at least one hour (or leave it in the fridge for 24-48 hours if you want the dough to be long fermented).
Take the dough out of the fridge and divide it into three parts.
Preheat oven to 350 degrees Fahrenheit.
On a floured surface, roll out a 1/3 of the dough to about a 1/4" thickness.
Take your biscuit cutter or mason jar lid and cut out circles.
Take cut-out circles and place them to the side.
Reshape dough into a ball and roll it back out. Repeat steps 9 and 10.
Place about one tablespoon of filling in the middle of each circle.
Now, you're going to assemble the hamantaschen into the traditional triangular shape. First, take the left side of the circle and bring it towards the center. Do the same thing with the right side and pinch them together. Then, lift up the bottom flap towards the two sides and pinch them together, making sure you can still see some of the filling in the middle.
Repeat for remaining circles and place on baking sheet.
Repeat steps 9-15 for the other two-thirds of the dough.
Place the hamantaschen in the oven, and bake them for about 11-12 minutes, until lightly golden brown.
Let cool for at least five minutes.
Enjoy your delicious sourdough hamantaschen!