Happy Fourth of July!
The earliest recipe I can find for these is from grocery ads in late 1940, like this example from the December 6, 1940 edition of The Ogden (Utah) Standard-Examiner:
Rice Krispies Marshmallow Squares
Melt butter and marshmallows in double boiler. Add vanilla; beat well. Put Rice Krispies in large buttered bowl and pour on marshmallow mixture. Press into shallow buttered pan. Cut into squares.
Yield: 16 2-1/2-inch squares (10 x 10 inch pan.)
Note: nut meats and coconut may be added.
From a box sold in East Moline, Illinois.
1/4 c. butter
1/2 lb. (32) marshmallows
5 c. Rice Krispies
Melt butter. Add marshmallows. Add Rice Krispies. [Stir] until coated.
9 x 13 inch pan.