Put sifted flours into the bowl of an electric mixer fitted with a dough hook. Add salt, sugar and yeast. Add beaten egg, butter, evaporated milk and lukewarm water. Mix and knead until dough is soft and elastic. Cover the bowl and leave aside to rise until doubled in bulk.
Remove dough onto a lightly floured tabletop. Knead lightly until smooth. Roll out to a rectangle with a thickness of 1cm. Stamp out with a triangular cutter. Use a small cutter and cut out the centre to make a hole. Leave doughnuts on a piece of nonstick baking paper to proof until doubled in bulk.
Heat oil and deep-fry doughnuts until golden and puffed up. Drain on absorbent kitchen paper towels. Drizzle over the orange zest syrup and serve.
While the doughnuts are proofing, prepare the orange zest syrup. Heat the orange zest, orange juice and sugar in a saucepan over medium heat. Stir until sugar has dissolved. Allow syrup to simmer for 8-10 minutes or until thick and syrupy. Leave aside to cool completely.