Cream butter and sugar together until creamy.
Add sourdough starter and mix until it’s incorporated with the butter and sugar mixture.
Mix in baking soda and salt.
Add egg and vanilla. Mix together.
Slowly stir in flour and nutmeg. Mixture should become combined into a thick cookie dough. Make sure to scrape sides of bowl during mix.
For more extra flavor and fermentation: Cover bowl and refrigerate 2-24 hrs. (Optional: leave it at room temp for 1-2 hrs to jumpstart the fermentation before putting the dough into the fridge)
Preheat oven to 400 degrees. Cut two sheets of parchment paper and place on two cookie sheets.
Lightly flour your work surface. Put chilled dough on top of flour then lightly flour the rolling pin.
Roll dough out until 3/8-1/2-inch. Cut out shapes, put shapes close together to get as many out as possible. (Tip: keep cookie cutters from sticking by coating cutting edge with flour.) Place cut out cookies on parchment paper.
Repeat as needed. You can chill cut out cookies on cookie sheet if they are becoming warm—the sign for this is if the cookies begin having a shiny look from butter softening. I recommend chilling cut cookies for 5-10 minutes before baking.
Bake 7-9 minutes until just a little bit golden around bottom edges and top of cookie is matte not glossy. Let cool on cookie sheets for 10-15 minutes to allow them to finish baking and firm up before enjoying.
Let cool completely before decorating.