Preheat the oven to 180°C.
Grease two 6-cup muffin trays with butter and add about ½ tsp caster sugar to each tin.
Slice the smaller orange into 3mm slices. Place the slices into the base of 10 muffin cups. Depending on the size of your orange, some slices may need one segment sliced out, then position into the muffin cup so that the rind appears joined again.
Coarsely chop and puree the whole large orange in a food processor. Transfer to a bowl and add melted butter, eggs, mixed spice and sugar. Mix well.
Sift the flour into the mixture and beat it well. Pour mixture into the 10-muffin cups, over the top of the orange slices. It should reach about halfway up the cups, allowing space to rise.
Bake at 180°C for 20-22 minutes. Cakes should be golden and springy to touch when they’re done. Remove from the oven and invert to a rack to cool.
