In a medium bowl, combine flour, baking soda, cornstarch, and salt. In a separate, larger bowl, beat butter and sugar with a hand mixer (or stand mixer) for a full three minutes, or until pale and fluffy. Beat in egg, followed by the vanilla.
Slowly add the flour mixture to the butter/sugar mixture until a smooth dough forms. It will be sticky; that is okay! Wrap in plastic wrap and place in fridge for at least an hour and up to a week in advance of making the bark.
When ready to bake, preheat oven to 350 degrees and line two cookie sheets with parchment paper. Roll cookie dough out to 1/4 inch thickness on a liberally floured surface with a rolling pin. Cut out with cookie cutters, and place on the prepared cookie sheet 2 inches apart to allow for spreading.
Bake at 350 degrees for 10-12 minutes (5-6 minutes if using mini cookie cutters) or until edges are just browned but centers are still soft and puffy.
Let cookies cool on cookie sheet for a few minutes before carefully removing to cool completely on a wire rack.
Once your cookies are cool, start making your bark! Line a baking sheet with aluminum foil. Add semisweet chocolate to a microwave-safe bowl and microwave in 20 second increments, stirring in between each session, until melted and smooth.
Repeat this process with the white chocolate, stirring in a bit of red or green food coloring once smooth (if desired).
Carefully pour melted semisweet chocolate onto foil-lined baking sheet, smoothing out with a spatula. Drizzle melted white chocolate over top, then gently press sugar cookies into still-warm chocolate.
Sprinkle liberally with rainbow or colored sprinkles, if desired. Let cool until chocolate is firm (place in refrigerator for 10-15 minutes if desired) then cut with a sharp knife into chunks and serve!