Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda.
In a large bowl, combine softened butter with sugar until fluffy. Add in egg and vanilla extract, and mix until smooth.
Add in the flour mixture, and mix until a thick dough forms. Fold in sprinkles. Press the cookie dough into a greased square baking dish.
Bake for 20-25 minutes. Cut into bars.