Knead 1kg of the icing until soft on a work surface lightly dusted with icing sugar. Make 3-4 thumb imprints in the icing and add a little gel food colouring (about the size of a small pea) to each. Knead the fondant until it’s no longer sticky and streaks of blue start to appear in the icing.
Dust the work surface again with icing sugar and roll out the icing to give a circle 35cm in diameter and 0.5cm thick – the marbling should now be quite pronounced.
Put the cake on a cake turntable (or straight onto a cake board/plate if you don’t have a turntable). Brush the marzipan lightly all over with water. Dust a rolling pin with icing sugar and use it to lift up the icing, then unroll it
over the top of the cake.
Smooth down the edges with your hands then, using a smoothing tool, smooth the surface of the cake.
Trim the bottom of the icing so it’s flush with the cake board/plate (keep offcuts for extra decoration). Leave to dry for 2-3 hours (see Make Ahead).
Roll out 250g white icing on a work surface dusted with icing sugar and use the snowflake cutters to cut out a few shapes. Brush one side with water, then stick the shapes on the cake in a pattern of your choice.
For the snowman decorations, take 3 pieces of the remaining 250g icing weighing about 30g, 20g and 15g and roll into balls. Push a cocktail stick into the top of the largest ball, then push the second largest onto the stick so the pick comes out the other side. Stick the smallest ball on top. Draw on eyes, nose (see tips), buttons and arms with the black icing pen.
Knead a tiny dot of blue colouring into 30g more icing and roll it out (or use the offcuts), then cut 2 x 8cm x 1cm strips to make scarves. Make another snowman in the same way using smaller balls of icing and half a cocktail stick.
Flatten the snowmen bases lightly, wet with water and stick onto the cake.