Preheat your oven to 180°C/160°C(fan)/350°F/Gas 4. Grease and line a 30x20cm (12x10") baking tin.
To make the cake
Add 250 g Butter and 250 g Caster sugar to a large mixing bowl. Mix together until light and fluffy.
Whisk in 4 Eggs one at a time.
Sift in 210 g Self-raising flour, 20 g Cocoa powder and 1 teaspoon Baking powder. Gently fold the dry ingredients in.
Add 140 g Plain yogurt, 1 teaspoon Vanilla extract and 50 g Chocolate chips. Carefully fold these into the mixture.
Pour the cake batter into the lined baking tin and smooth over. Bake for 30-40 minutes, until an inserted knife comes out clean.
Once cooked, leave the cake to cool for at least 10 minutes.
Slice the cake in half through the middle and spread 100 g Jam over the bottom half.
To make the buttercream
Mix together 250 g Butter, 400 g Icing sugar, 2 tablespoon Milk and 1 tablespoon Vanilla extract, until smooth.
Remove two small amounts and put in separate bowls.
Add 15 g Food colouring gel (green) to the large amount and 5 g Food colouring gel (red) and 5 g Food colouring gel (yellow) to the smaller amounts. Mix well to get the desired colours.
To make the cake
Pipe about ⅓ of the green icing onto the jam. Put the top half of the cake back on and cut out a tree shape.
Starting from the bottom of the tree, pipe the remaining green icing in small downward motions, working upwards and slightly overlapping the row below.
Add small spots of red and yellow icing to make lights and baubles.
Notes
Note that the nutritional information will be higher than actual, as not all of the cake is used, depending on how you cut the tree shape.
You could swap the icing for a cream cheese frosting if you prefer.
Allow the cake to cool fully before slicing it in half and adding the filing. It will fall apart otherwise.
You could use a mini star shape cookie cutter to cut out a star from the off cuts of cake to put on top of the tree.
We like to dust it with a little icing sugar before serving to make it look a little snowy.