Finely grate the rind of 2 of the oranges. Using a sharp knife, cut the tops and bottoms from all the oranges and remove the skin and pith. Hold the oranges over a bowl to collect the juice and cut out the segments.
Melt 50 g (2 oz) of the butter in a frying pan, add the caster sugar and 100 ml (3 1/2 fl oz) of the reserved orange juice and half the rind and cook over a medium heat, stirring occasionally, until the sugar has dissolved. Simmer for 4–5 minutes until thickened and syrupy. Stir in the orange segments to heat through.
Meanwhile, mix the remaining butter and orange rind with the icing sugar and cinnamon. Arrange the tortillas on a large baking sheet, then brush one side with the butter mixture. Place in a preheated oven, 200ºC (400ºF), Gas Mark 6, for 2–3 minutes or until golden. Leave to cool slightly, then cut each into 4 triangles. Spoon the oranges and syrup into bowls and serve with the tortilla triangles.