Mix yeast with sugar and dissolve in milk. Mix with flour, egg yolks, butter, salt and lemon zest. Knead using the dough hook of an electric hand mixer into a soft dough. Knead until dough pulls away from edges of bowl. Cover and let rise in a warm place until volume has doubled.
Knead dough vigorously on a floured surface. Roll out until about 1 cm (approximately 1/2 inch) and cut into squares about 10 cm (approximately 4 inches) long. Cover and let rest for 30 minutes.
Place dough squares top side down in hot oil at 180°C (approximately 350°F). Turn once top side is brown.
Remove doughnuts with a skimmer and drain on paper towels. Cool slightly and then sprinkle with powdered sugar to serve.