Remove the grate from the stove top. If you've cooked recently, wait for it to cool before handling it.
Fill the sink with soapy water. It should be warm to the touch.
Dip your sponge in the soapy water, wring it until it's damp and run it over the surface of the grate to remove any loose white powder.
Mix a little water and baking soda together in a container until you've formed a paste. Rub the paste onto any powder that isn't coming off the grate. Allow the paste to dry for 15 minutes.
Rub the grate with a non-scratch pad to remove the baking soda paste and white powder.
Rinse the grate under warm, running water. Allow it to dry completely before returning it to the stove.