Grease heavily the back of a big pizza pan or thali and rolling pin or alternately tape parchment paper to a tray or countertop. (See notes)
Dry roast sesame seeds in a pan on medium-low flame for 3-4 minutes with constant stirring until lightly toasted. Remove toasted seeds to a plate and set aside.
Keep a bowl of water ready for testing jaggery caramelization.
Heat oil in a wok, then add grated jaggery. Melt jaggery on medium flame, while stirring constantly.
Now increase the heat to medium-high so jaggery starts bubbling and caramelizing, stir well so jaggery is uniformly caramelized.
Test jaggery caramelization by dropping a drop of melted jaggery in the water. Press it. If the drop forms a soft lump, cook more. If it turns hard immediately then your jaggery is ready.
Mix in roasted sesame seeds and quickly pour all mixture on greased thali or parchment paper. With a greased rolling pin make ⅛ inch flat sheet.
Make cutting marks while the brittle is still hot. Let the chikki cool completing before separating.
Store in airtight container at room temperature.