Preheat the oven to 180°C/350°F/Gas 4.
Put the sugar in a bowl and break up with the back of a fork to ensure all the lumps of sugar are out – soft light brown sugar often has small darker lumps in it so you might want to sift it first. Beat in the oil and eggs. Fold in the grated squash and lemon zest. Combine the flour, baking powder and cinnamon in a bowl, then sift into the mixture and fold in.
Spoon the mixture into the paper cases and bake in the preheated oven for about 18 minutes until risen and a skewer inserted in the centre comes out clean. Transfer to a wire rack to cool.
To decorate, put the white and dark chocolates in separate heatproof bowls set over pans of gently simmering water – I melt my chocolate in a heatproof bowl over another heatproof bowl of water boiled from the kettle and it works just as well. Do not let the bowls touch the water. Leave until almost melted. Remove from the heat and leave to cool for about 5 minutes, then spoon the white chocolate over the cakes.
Cut a large square of a greaseproof paper and fold into eighths to make a cone and tape together – or you could just use a disposable piping bag which is so much easier and saves you worrying about chocolate squeezing out everywhere. Spoon the dark chocolate into the cone and snip the tip off so that you can pipe a thin line of chocolate. Put a dot of chocolate in the centre of each cake, then pipe three concentric circles around the dot.
Using a skewer, draw a line from the central dot to the outside edge of the cake and repeat about eight times all the way round to create a spider’s web pattern. Serve while the chocolate is still slightly soft and gooey.