Heat the water, butter and salt together in a large heavy saucepan or dutch oven.
When the butter is melted and the water begins to boil, reduce the heat, then add the flour all at once. Stir until a ball forms. Remove from the heat.
Add the eggs, one at a time, beating until each egg is completely incorporated into the dough before adding the next. (You'll be able to tell. With each egg the dough becomes shinier and starts to look like a really thick batter.)
Set the batter aside while you heat oil in a medium saucepan. You'll want the oil to be about 2 inches deep. If you have a thermometer the oil should be between 360°F and 375°F (182°C to 190°C). Otherwise, the oil is ready when it shimmers and a small amount of dough sinks and then immediately rises back to the top. The temperature is important because the steam that forms inside the buñuelo helps to cook it from the inside. If too hot, it will brown too quickly and the interior will be raw. Too cool, and they'll be greasy.
Use a couple of spoons to portion the dough and fry in batches, turning once or twice. The dough will puff up as it cooks. The time it takes to cook depends on the size of the fritters and the temperature of the oil. Mine took 3 - 4 minutes.
Drain the cooked buñuelos and toss or sprinkle with cinnamon sugar.
Serve right away or use a pair of kitchen scissors to cut a small hole and fill with the pastry cream.
These are best eaten right away or at most the day after. They'll get soggy, but will still taste good.