Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, mix together butter and sugar until fluffy. Add in egg and vanilla extract and mix until smooth. Add in the flour mixture and mix until it is fully combined.
Press the cookie dough into a greased 8-inch square baking dish and bake for 20-25 minutes. Let cool completely. In a large bowl, mix together butter, powdered sugar, and vanilla extract until smooth.
Spread the frosting onto the cooled cookie bars. Decorate with sanding sugars and cut into squares.
