Make the cake: Preheat the oven to 350˚ F. Butter two 8-inch round cake pans and dust with flour. Sift the flour, granulated sugar, cocoa powder, baking powder, baking soda and salt in a large bowl. Add the eggs, vegetable oil, buttermilk and vanilla and mix until combined.
Dissolve the espresso powder in the boiling water in a mug, then pour into the batter and mix until well combined.
Divide the batter between the cake pans. Bake until a toothpick inserted into the center of each cake comes out clean, 25 to 35 minutes. Let the cakes cool 10 minutes in the pans, then remove them to a rack to cool completely.
Meanwhile, make the frosting: Beat the confectioners' sugar and egg whites in a stand mixer fitted with the paddle attachment on medium-high speed for 1 to 2 minutes. Slowly add in the butter, a tablespoon or two at a time, and continue mixing. Once combined, increase the speed to high and beat until smooth. If the frosting looks curdled and separated, remove one-quarter of it to a microwave-safe bowl and melt in the microwave for about 10 seconds, then pour back into the rest of the frosting and whip on high. This should bring the frosting together; if not, repeat this process again. Stir in the vanilla and salt.
Add the chocolate-hazelnut spread to the frosting and beat until combined. (This step is optional — you can just keep the frosting vanilla if you’d like.)
Crumble up the cakes in a large bowl. Add 1/2 cup frosting and mix with your hands. You should be able to form the mixture into a ball but it shouldn’t be too soft.
Make the topping: Melt the colored candy melts in the microwave as directed. Assemble the cakesicles in batches: Add a little (about 1 tablespoon) melted candy melts to each pop mold (see box below). Brush the candy around the mold to cover completely. Slide a pop stick into each mold and chill until the candy hardens, about 10 minutes. Add a second coating of melted candy melts if needed.
Form 3 to 4 tablespoons of the crumbled cake mixture into a ball, then press into one of the molds, adding more cake mixture as needed to fill. Repeat to fill all the molds, then spread a layer of melted candy melts on top to seal it all in. Refrigerate until set, 15 to 20 minutes. Carefully remove each cakesicle from the mold and repeat with the remaining cake mixture and colored candy melts (remelt them if needed).
Melt the white candy melts in the microwave, then drizzle on the cakesicles. Decorate with sprinkles.
You’ll need flexible 3- to 4-inch pop molds to make these. Be sure to get the kind used for cake pops or ice cream, not upright ice pop molds. They often come in sets of two 4- or 5-cavity trays.