Put plain flour, rice flour, egg,coconut milk,sugar and salt in a mixie and blend just until smooth and without lumps. You can also Mix it well using a wooden spoon or manual whisk. Add sesame seeds and Kalonji (if using)and stir well. Transfer some batter to a small and deep vessel. Heat oil in a deep and wide pan. Place the Achappam mould in it, while heating the oil. Once the oil is heated enough, it should be really hot, but not smoking hot, dip the heated achappam mould in the small vessel filled with batter. Make sure you dip only 3/4 th of the mould in the vessel, otherwise the batter may stick to the mould and and achappam won't slip out of the mold as as it should be. Immediately dip the mould in hot oil and shake it a little so that the batter loosens into the oil. While the achappam is frying, place the mould in the oil (refer notes). Once it start getting brown around edges, flip the achappam using a slotted spoon and cook till both sides turn brown. Keep in mind the colour of Achappam tends to darken and hold it's shape as it rests. Drain the fried Achappams on paper towel. Once it's cooled completely, store in an airtight container for up to 1 week at room temp