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.
Notes
I used 3.5 tbsp sugar and felt it was sweet enough. If you prefer a sweeter version, add 4 tbsp. But make sure you dont add more sugar, because if there is too much sugar, the batter might stick to the mould.Adjusting the heat is very important in frying Achappams, Heat the oil in med-high flame, once it's really hot, reduce it to low-medium and fry.Though the Achappams colour darken, it might appear to be soft, it will hold it's shape and crispiness as it rests.You can also make it with just rice flour, in that case use 1 cup rice flour and follow the rest of the recipe. You may need to mix the batter in a blender. I tried both versions (just rice flour and a combination of plain flour and rice flour). We liked the the combo one better (mix of plain flour and rice flour).Make sure you dip the mould in hot oil, each time, for a few minutes, before dipping it in the batter. Transfer more batter to the smaller vessel as and when required.To season a new mould - wash it really well with soap and water, dry with a dish towel and immerse in really hot oil and keep it aside for a while. If you havent used your mould for a long time, soak it in tamarind water for 1-2 days(if it's brass mould do not do this step). If you are using a brand new mould, make sure you season it before using. After using the mould, wash and dry it and grease it with some oil.If you do not have an Achappam mould, use a brass cookie cutter. Make sure you dip it in oil, using tongs and follow the rest of the recipe. The shape may be different, but you still get the taste of homemade achappam. See the picture below: