In a medium bowl, whisk together the egg, flour, buttermilk, sugar and a pinch of salt. Add the soda water gradually until you make a thick smooth batter. Set the batter aside to rest for 15 minutes.
Peel the plantains by making a slit with a knife, the long way, from one end to the other, and cutting off the ends. Pull the skin off.
Slice the plantains diagonally into slices about 3 inches long and about 1/3" thick.
In a heavy pot, or deep fryer heat up vegetable oil to a minimum depth of 2 inches. Heat up to 350 degrees or until a drop of batter sizzles.
Once the oil is hot, dip the plantain slices into the batter to coat them and carefully drop them in the oil taking care not to over crowd the pan.
Cook until golden brown on both sides turning once, about 3-4 minutes. Remove with slotted spoon and let drain on a paper towel to remove excess oil. Sprinkle with salt and serve warm with the spicy peanut sauce and a lime.