2 pastry/piping bags; couplers for piping bags; piping tips (small grass tip #133, small star tip #32, plain tip #6); flexible cardboard or cardstock; snowflake cookie cutter
Using a paring knife, start carving the top edge of your cake to round it off. Continue carving about an inch down the cake to create a rounded hat shape. Crumb coat in a thin layer of buttercream. Use the flexible cardstock to smooth over the curved edge to help the cake maintain its shape. Refrigerate for 20 minutes.
While chilling, mix three-quarters of your buttercream in a bowl with a few drops of blue food coloring. Put into a pastry bag with a coupler and a small star tip. Put the remaining white buttercream into a pastry bag with a coupler and a plain tip.
Take the cake out of the refrigerator. Use a snowflake cookie cutter to imprint outlines and make templates for the pattern onto the sides of the cake. Starting with inside your imprint lines, use the white buttercream to fill in the shape with dots of buttercream.
Once all your snowflakes are filled in, use your blue buttercream tip to create a herringbone pattern around the whole hat from top to bottom. Add a braided teardrop shape to accent.
Pipe a star tip shell border to represent the brim of the hat.
Roll your white fondant into a ball and place it on the top of your hat. Using the small grass tip, pipe long "fur" onto the fondant to completely cover it.