Preheat the oven to 180c.
Into a small bowl, zest one of the oranges. Set aside.
Peel and slice all four oranges width ways into 1cm wide discs. Set aside.
Grind the cardamom seeds in a mortar and pestle or a spice grinder until they are roughly broken up (they should look like roughly ground black pepper).
In a measuring jug, whisk together the soy milk and the apple cider vinegar. Set aside for at least ten minutes to thicken slightly.
In a medium bowl combine the flour, baking powder and salt.
In the bowl of a stand mixer, beat the butter and caster sugar on a high speed until softened. Add the vanilla and orange zest and beat to combine.
Reduce the stand mixer speed to low and add a quarter of the soy milk mix, followed by a quarter of the flour mix. Add another quarter of the soy milk and continue to alternate with the flour until all ingredients are added. Do not overmix.
Remove the cake tin from the freezer and sprinkle an even layer of the ground cardamom over the now frozen caramel. Evenly spread the orange slices on top until there are no large gaps of exposed caramel. Working quickly, pour the cake batter over the orange slices and tap the cake tin on a solid surface to release any trapped air bubbles.
Bake for 35 minutes. Before removing, make sure a toothpick to the centre of the cake comes out clean.
Allow to cool for 20 minutes before inverting the cake onto a plate and serving.