1. Preheat your oven to 350°F (180ºC). Start by making your caramel. Put the sugar and water in a small saucepan. Stir to dissolve the sugar and bring to a boil. Bubble until it turns a light caramel colour.
1. Working quickly, pour the caramel into a round 9-inch cake tin. Carefully place the blood orange slices on the bottom of the pan. The caramel is still hot, so be careful!
2. In a large bowl, cream together the butter and sugar until light and fluffy, about 5 minutes. Add the eggs, one at a time and mix until fully combined. Mix in the vanilla.
3. In a small bowl combine the flour, baking powder and salt. Mix in the dry ingredients and the yogurt. Mix until fully combined.
4. Place in the oven for 30-35 minutes, until the cake is cooked through and springs back when touched. As soon as the cake comes out of the oven, turn it out onto a wire rack. If you leave it, the caramel will set and it will be really difficult to turn the cake out!