Preheat the oven to fan 160C/conventional 180C/gas 4. Butter and line an 18cm round, 7cm deep cake tin with greaseproof or parchment paper.
Put the flour, bicarbonate of soda and all the spices into a large mixing bowl. Add the butter and rub it into the flour with your fingertips until the mixture resembles fine breadcrumbs.
Put the sugar, treacle, syrup and milk in a medium saucepan and heat, gently stirring until the sugar has dissolved. Turn up the heat and bring the mixture to just below boiling point.
Add the stem ginger to the flour mixture, then pour in the treacle mixture, stirring as you go with a wooden spoon. Break in the egg and beat until all the mixture is combined and it resembles a thick pancake batter. Pour this into prepared tin and bake for 50 minutes-1 hour, until a skewer pushed into the centre of the cake comes out fairly clean. Leave to cool completely in tin before turning cake out. (To freeze: wrap in greaseproof paper, then in cling film. Freeze for up to 1 month.)
To make the icing, mix together icing sugar and lemon zest, then gradually add lemon juice until you have a smooth, slightly runny icing, adding more juice, if needed. Drizzle icing in a zig-zag pattern over surface of cake, turn cake around and drizzle again to create the cross-hatched finish (see below). Cake keeps for up to 2 weeks stored in an airtight container.
Prepare your cake by stacking your layers and filling with jam and buttercream icing, then place on a cake turntable.
Measure across the top and sides of the cake using a piece of string. This will act as a guide for when you roll the icing out later.
Create smooth edges and sharp corners by turning the cake on the turntable and simultaneously using a palette knife to smooth the buttercream on the sides and top, scraping off any excess as you go. Leave it in the fridge to firm for 30 mins.
Make sure your work surface is clean and free of any crumbs. Dust liberally with cornflour or icing sugar. Knead fondant icing for a couple of minutes to soften, or pop it in the microwave for 10 seconds.
Flip the smooth side to the top, then roll it out to fit the cake, using the string for guidance. Lift the icing using a rolling pin or your hands; drape it over the top of the cake. Smooth the fondant using cake smoothers, pushing out any air bubbles and creases, starting on the top then working down the sides.
Use a small knife to cut away the excess icing – don’t cut too close to the cake. Save the offcuts for decoration. Polish using two smoothers together for a perfect finish.