Allow the layers to cool. Once completely cool (refrigeration helps with getting the cakes cool enough to cut), carefully slice off the top "dome". (Reserve this top "dome" to snack on, or make cake pops!)
Choose which cake layer will be on the bottom, and place it on top of a flat plate, cardboard circle, or other stiff surface. Remember, the more you build, the more difficult it will be to move once it's complete, so you want it on something stiff that will help easily slide it back and forth, rather than building it on a textured surface like a cutting board or a kitchen counter, or something with a lip like a standard dinner plate.
Fill a piping bag with buttercream. Working in a spiral motion from the inside out, pipe the buttercream on the top of the first layer of cake. It's okay if it isn't perfect and if it spills out to the sides - we'll end up using that anyways.
Choose the next layer of cake and repeat the process.
Take the final layer of cake and flip it upside down, and set it on top of your construction. Flipping it upside down ensures a smooth, flat top to your top layer, as it's molded to the bottom of your baking pan.
Add another spiral of frosting on top of the cake. Pipe more frosting in between each layer.
Using a large offset spatula, smooth the top of the cake frosting, and then the sides. Pipe more frosting on the sides of the cake as needed, and smooth with the spatula.
To create the white chocolate snowflakes, draw a snowflake pattern on a piece of parchment paper with a heavy pencil or marker.
Melt the white chocolate and pour it into a squeeze bottle or piping bag fitted with a thin tip.
Flip the parchment paper over so you can still see the snowflake pattern through it, and trace the pattern with the white chocolate.
Allow the snowflakes to cool completely, either on the counter or in the refrigerator, and carefully pull the parchment away.
Decorate the finished cake with the white chocolate snowflakes, powdered sugar "snow," and frosting rosettes or stars.