Preheat the oven to 190°C.
Grease a medium bundt cake tin, then dust with flour.
In a small bowl, whisk together the yogurt, oil and vanilla, then add the orange zest and juice.
In another bowl, mix the flour, baking powder, bicarb and sugar. Pour in the wet mixture gradually, then stir until well combined.
Transfer the batter into the tin.
Put the tin in the middle of the oven and bake for about 50 minutes or until an inserted skewer comes out clean.
Let it rest in the tin for 10 minutes, then turn it onto a wire rack and let it cool completely.
Meanwhile, cut the orange into thin rings. Heat a frying pan and melt the margarine. Dust with sugar then put the orange slice over the sugar.
Caramelize the slices for a minute or so, then scatter some more sugar on top and turn them over.
When the orange slices are golden on both sides, transfer them to a plate and let them cool.
Make the glaze by combining the icing sugar with the orange juice. Pour this over the cake, then “glue” the orange slices on top of the cake.