Crumble the cooled cake into the bowl with the buttercream. Use your hands or a standing mixer to mix it all together until fully combined. If using store-bought frosting, begin by using 2 tablespoons, then add one more if the mixture seems dry. The amount of frosting will vary depending on the type of cake you are using. Set aside.
If your cakesicle molds are new, the holes for the stick might be covered, so poke a stick through them before beginning to assemble the cakesicles.
Melt the chocolate. You can melt it in the microwave for 30 seconds at a time, stirring between each time. You can also melt the chocolate over low heat in a double boiler or pan on the stove.
Add 1 tablespoon of melted chocolate into each cakesicle mold cavity, using a spoon to spread it up the sides of the mold. Stick a popsicle stick into each one then remove it - this is to make sure the chocolate doesn't set and harden over top of the hole. Refrigerate the molds for 5-10 minutes or until chocolate has set.
Remove the cakesicle molds from the fridge. Check along the sides of each mold to make sure the chocolate coating doesn't look too thin. If it's too thin, add another layer of melted chocolate onto the sides and refrigerate for another 5 minutes or until set. Next, fill each cakesicle mold with your cake mixture, flattening and smoothing the top. Don't fill them all the way to the top - leave a little bit of room on top of the cake for the melted chocolate.
Insert a popsicle stick through each cakesicle hole. Top with more melted chocolate to cover the cake. If you want to make it really smooth, use an offset spatula to scrape off the excess chocolate and clean up where it has spilled over. Refrigerate the molds for 10 minutes, or until chocolate has set.
Line a large baking sheet with parchment paper. Once the cake popsicles have set, remove any hardened extra chocolate around the molds then gently pop the cakesicles out of the molds. Try not to push the popsicle sticks too hard - you don't want them to pop out of the top of the cakesicles. Instead, gently peel back the silicone around the cake pop to get it out.
Place the cakesicles onto the baking sheet. Drizzle more chocolate in a zig zag pattern over each one and add crushed candies or sprinkles, or decorate as desired! There are decoration tips further up in this post for some fun ideas!