1Preheat the oven to 180C/ 160C Fan/Gas 4. Line the bases of 4 x 18cm round sandwich tins with baking paper (do not grease).
2Sift the flour, baking powder and bicarb into a large bowl and stir in 275g of the caster sugar.
3In a separate bowl, beat the butter with an electric whisk until smooth. Add ½ the soured cream and the vanilla extract, then beat again until smooth. Stir through the remaining soured cream. Set aside.
4Clean and dry the beaters thoroughly. Then, in a third bowl, whisk the egg whites to soft peaks. Turn the whisk to low. Add the remaining caster sugar a little at a time while continuing to whisk. Beat to firm peaks.
5Add the soured cream mixture to the bowl with the dry ingredients then mix together. Gently fold through the egg whites, a few tbsp at a time, until no streaks of white remain.
6Divide the mixture evenly between the 4 tins, then smooth and level the tops.
7Bake for 20-25 mins, until the cakes spring back when lightly pressed in the centre. Cool for 20 mins in the tins before turning out to cool completely on wire racks.
8To decorate, put the icing sugar and butter into a large bowl. Beat together into a smooth buttercream.
9Put ⅓ of the buttercream in a separate bowl. Stir in the peppermint extract and food colour until you get a uniform green shade.
10Stir the vanilla extract into the white buttercream.
11Put about ½ of the green buttercream and about ⅓ of the white buttercream into separate piping bags (no nozzles required). Cut 1cm off the tip of each bag.
12Put the bottom cake on the cake stand. Pipe a large ring of white buttercream on top, near to the edge of the cake. Next, pipe a ring of the green buttercream directly inside the white one. Continue to pipe, alternating white and green, until the top of the cake is completely covered.
13Pipe more buttercream rings on a second cake, this time starting with green on the outside. Carefully lift it onto the first cake. Repeat with the third cake, starting with white buttercream. Add it to the stack then top with the fourth cake.
14Cover the top and sides of the cake with the rest of the white buttercream, smoothing with a scraper or palette knife. Chill the cake in the fridge for 15 mins.
15For the Christmas trees, trim 2-3cm from the base of a few of the waffle cones with a sharp breadknife, so they vary in height.
16Slice 2 different-sized cones in half lengthways. Position the halves against the side of the cake, with the wider ends sitting on the cake stand. Push the cut sides into the buttercream to secure in place.
17Fit a piping bag with the star nozzle; fill with the rest of the green buttercream.
18Starting with the halved cones on the side of the cake, pipe on ‘kisses’ of the green buttercream. Work your way up from the wide base to the tip. Arrange a cluster of 3 different-sized cones on top of the cake and pipe as before to cover.
19Shortly before serving, dust the cake and trees lightly with icing sugar to create a snowy effect.