STEP 4: Let the sheets sit.
Let the paste-covered pan sit undisturbed for about two hours—usually enough dwell time for the mixture to break away at the burnt-on crud.
STEP 5: Gently scrub the sheets.
Run some warm water over a household sponge (not an abrasive pad, which could scratch non-stick surfaces) and start to gently scrub the pan. The brown grime should be significantly loosened. Some tough spots still might take a bit of elbow grease, but most of the gunk should come off easily. Scrub until the pan is sparkling, then rinse with warm water.
STEP 6: Thoroughly dry the sheets.
Immediately dry the clean pan thoroughly. This step is especially important—skip it, and you run the risk of developing rust or corrosion.
STEP 7: Repeat steps on other side of sheets.
If the back of the cookie sheet pan is also burnt and stained, repeat all steps on the other side.
Now you’re ready to bake some more cookies. Be sure to set a timer for this batch! It may do you well to also consider laying down some parchment paper rather than a thin coat of cooking spray; the former won’t leave behind any sticky residue to burn and bog down dish-washing.