In a large bowl, beat together butter and sugar until light and fluffy.
One at a time, beat in the egg and then vanilla extract.
Mix in flour and baking soda until just combined (do not over mix -the dough should not be sticky). Add an additional 1 - 2 tablespoons of flour if needed.
Prepare the dough for refrigeration: pat the dough into 2 flat cakes. Wrap each one separately in plastic wrap and chill them for two hours in the fridge.
After the dough has chilled, work with one portion at a time and roll it out on a lightly floured surface. Use a rolling pin to roll out the dough until it is 1/4" thick.
Dip a cookie cutter in flour and cut out heart shaped cookies. Place them 1" apart on a buttered baking sheet.
Bake in the oven preheated to 350 degrees 10 - 12 minutes, until the edges turn pale golden brown.
Cool on a wire rack and store in airtight containers until ready to ice and decorate.