Combine flour, baking powder, sugar, cardamom and salt in a large bowl.
Whisk egg and water together in a small bowl. Make a well in center of dry ingredients and add egg mixture. Mix together gradually with a fork until mixture forms a soft dough. If it's too sticky, add 1 or 2 tablespoons of flour (one at a time).
Cover dough with a wet towel or plastic wrap and leave 30 minutes or longer.
Heat 4 cups of oil in a deep, heavy pot (cast iron is preferable) until is reaches 360°F
While oil is heating, roll dough out on a floured surface into a rectangle ½ inch thick. Use a sharp knife to cut into 20 equal rectangles.
Fry maandazi in batches of 5, for about 5 minutes per batch, turning to brown both sides. Hint: Turn the mandaazi before it gets too puffy, or the air bubbles will prevent you from turning it at all.
Remove from oil when both sides are golden brown. Serve warm, dipped in powdered sugar if desired.