Place chocolate wafer cookies in a resealable plastic bag and break into small crumbs with a rolling pin. Add cake sparkles and shake to combine.
Using a serrated knife, trim dome from top of cake and invert onto serving platter so flat side is up. Coat with chocolate frosting, spreading smooth with an offset spatula. Cover top and sides of cake with chocolate cookie-crumb mixture, pressing gently to adhere. (Place platter on a wire rack set over a rimmed baking sheet to contain the mess.)
On a work surface that's been lightly dusted with confectioners' sugar, roll out small pieces of fondant to about 1/8" thick. (Work one color at a time, and keep others well wrapped while not in use.) Cut circles using assorted round biscuit cutters.
Use a paring knife to cut larger circles into starburst shapes.
Place the fondant starburst pieces on top of the prepared cake, adhering with a dab of chocolate frosting. Add a smaller circle on top of the bursts. Arrange lollipops on surface, inserting some of the sticks directly into the cake. Sprinkle with silver sanding sugar.