Melt the butter, sugar, treacle and syrup in a small saucepan, whisking until smooth. Set aside to cool for 10 mins. Heat oven to 160C/140C fan/gas 3. Grease and line a 2lb loaf tin with baking parchment (our tin was 22 x 7 x 7cm).
In a bowl, mix the flour, bicarbonate of soda, ground ginger, cinnamon and 1?2 tsp salt. Pour in the warm syrup mixture and the milk, followed by the egg yolks. Whisk until you have a smooth batter. Pour into the tin and bake for 1 hr or until a skewer comes out clean. Cool in the tin for 30 mins, then slice off the top to give you a smooth surface. Flip onto a wire rack and remove the baking parchment. Leave to cool. Can now be covered with cling film and kept for up to 5 days.
To make the buttercream, put the ingredients in a large bowl and mash together with a fork or spatula (this prevents the icing sugar from going everywhere). When roughly combined, blend with an electric hand whisk until smooth.
Now make the marshmallow icing. Put the egg whites, sugar, 1 tbsp water and a good pinch of salt into a heatproof bowl. Place over a pan of simmering water, making sure the bowl doesn’t touch the hot water, and whisk until thick and leaving a prominent trail from the beaters – this will take about 4 mins. Remove the bowl from the heat and continue whisking for another 3 mins until cooled slightly and really stiff. Transfer half to a piping bag fitted with a large round nozzle and set aside.
When ready to assemble, slice the cake lengthways into 3 even layers. On a cake stand or plate, reassemble the cake, with the largest slice on the bottom, layering with the buttercream.
Using a small offset palette knife or small butter knife, cover the cake with the marshmallow icing left in the bowl. Use the palette knife to create vertical lines along the edges (see tip, left). Now use the piping bag to pipe spikes of meringue over the top of the cake. Use a cook’s blowtorch to toast the meringue to a golden brown – a few charred patches will add to the effect. Serve straight away or chill for up to 2 days, removing from the fridge 30 mins before serving. We decorated ours with sparklers.