Cut the oranges into slices about half a centimetre thick, removing any seeds.
Bring the sugar and water to a simmer in a large saucepan and add the orange slices. Simmer for 15 minutes, turning occasionally to make sure they are all fully submerged.
Remove the slices from the syrup and set aside to cool. Discard the syrup.
Grease a 23cm loose-based cake tin and line the bottom with baking parchment.
Arrange the orange slices in the tin, starting from the centre and working outwards, overlapping them slightly until all the slices are used up.
To make the cake:
Preheat the oven to 180C (fan) / 350F / Gas Mark 4.
In a large bowl combine the flour, bicarbonate of soda, caster sugar, ground almonds and cornflour.
In a jug, mix the oil and vegan yoghurt, then add the zest of the remaining orange (you will use the juice for the orange syrup).
Mix the wet ingredients into the dry until completely smooth.
Pour the batter into the tin over the orange slices, and gently smooth it to the edges of the tin.
Bake for 30 minutes or until an inserted knife or skewer comes out clean.
To make the orange drizzle syrup:
In a small jug or bowl mix the juice of the orange with the icing sugar.
Microwave for 1 minute on full power (or simmer in a saucepan until slightly thickened) then set aside to cool for 5 minutes.
Use a cocktail stick or skewer to prick the cake all over (whilst it is still in the tin), then spoon over the orange syrup, reserving about 1 tablespoon of syrup.
When the cake is completely cool, turn it out onto a serving plate. Remove the baking parchment and brush the orange slices with the remaining orange syrup.
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Notes
If you have time, leave the cake overnight in an airtight tin or container before serving. It makes a big difference to the texture of the cake, drawing the moisture right through the cake for a very soft and moist sponge.Once you have mixed the cake batter together, work as quickly as you can to get the cake into the oven. The raising agents start to work straight away, and you want the bubbles to form in the oven, not whilst sitting on the side!