I hope this is the one your father-in-law remembers. Good luck.

Atomic Cake

3 c. flour
2 c. sugar
6 T. cocoa
1 t. salt
2 t. soda
3/4 c. vegetable oil
2 c. water
2 t. vanilla
2 T. vinegar
2 dozen large marshmallows

Grease 9x13 pan and place marshmallows on bottom. Mix all other ingredients and
pour over marshmallows. Bake 45 minutes at 350 degrees. After cake cools: cover
to keep top soft, or uncover to make chewy.