Roll out cookie dough; trace cake and hat patterns with toothpick and cut cookies using knife. Cut 6 heart cookies using 3rd smallest cutter. Bake and cool all cookies. Cover heart cookies with thinned buttercream; sprinkle with colored sugars and let set.
Ice cake sides, tummy and background areas smooth. Outline body, snout, mouth and eyes with tip 3. Cover bear with tip 16 stars. Pipe in eyes, nose and mouth with tip 3 (smooth with finger dipped in cornstarch); add tip 2 string eyelids and dot freckles. Outline and pipe in lollipops on tummy with tip 3; add tip 3 string confetti on cake sides. Pipe tip 21 shell bottom border.
Cover hat cookie with tip 6 stripes; position on cake. Add tip 3 zigzag brim and pull-out string pompom. Ice cake cookie smooth. Pipe tip 6 stripe bottom border. Add tip 3 dot and string garland. Ice top of cookie fluffy with spatula; add tip 3 dot drips of icing. On round candle, insert tip 16 over wick and pull out icing flame. Insert candle in iced cake cookie top; position in hand area of cake. Attach curling ribbon to heart cookies with dots of icing; position around cake. Attach ribbon ends at top of hand and shorter ribbon strips at bottom with icing.