Blush orange upside-down cake with Grand Marnier caramel

Preheat the oven to 180ºC, gas mark 4. Grease and line the base and sides of a 23cm round springform cake tin with baking parchment and set aside. Sprinkle 100g sugar into a small frying pan with 2 tbsp water and put over a medium-low heat. Allow to melt, without stirring, then increase the heat slightly and bubble to a golden caramel (about 5-8 minutes). Tilt the pan occasionally so that the sugar melts evenly.

Remove from the heat, then add 50g butter and the Grand Marnier and allow to melt (take care as it may spit). Scrape into the tin and tilt to cover the base evenly.

Slice the top and bottom off all the oranges (be sure to zest 2 of them first) then, using a small, sharp knife and working from top to bottom, peel and remove the pith, following the curve of the fruit. Cut each orange into 0.5cm slices, then, discarding any seeds, arrange the slices on top of the caramel, sitting closely side by side and in a single layer. Any remaining slices can be cut to fill gaps.

Mix the ground almonds, flour, baking powder, salt and orange zest in a bowl and set aside. In a large bowl, use an electric hand mixer (or a freestanding mixer) to beat the remaining 200g butter and 130g sugar on high speed for about 3 minutes until light and fluffy. Add the eggs, one at a time, then reduce the speed to medium and add the yogurt and vanilla. Whisk just to combine, then use a spatula to fold in the dry ingredients

Scrape the batter into the tin, then put on a baking tray and bake for 35-45 minutes until golden brown and a knife inserted into the middle comes out clean. Remove from the oven; set aside for 10 minutes before releasing from the tin. Turn out onto a platter; set aside until completely cool. Serve with yogurt and toasted flaked almonds (if using).