Preheat the oven to 350°F (180°C). Grease and line a 9×13-inch (20x30cm) baking pan with parchment paper.
Cream the butter and sugar together in a mixer until light and fluffy. Add the egg and vanilla extract and mix until well combined.
Sift the dry ingredients (flour, salt, and baking soda) and gradually add them to the wet mixture. Mix until just combined.
Spread the dough evenly into the prepared pan and press down gently. Sprinkle granulated sugar on top if desired.
Bake in the preheated oven for 20-25 minutes, or until the edges are lightly golden and the center is set.
Cool the cookie bars on a wire rack.
Prepare the frosting: Beat together the butter, powdered sugar, vanilla extract, and salt until smooth. Add cream/milk until the frosting is spreadable. Add food coloring if desired.
Frost the cooled bars with the buttercream frosting. Decorate with sprinkles if desired.
Chill the bars for a few minutes to help set the frosting before cutting them into squares or rectangles.
Serve and enjoy!