I'm very fond of Wacky cake as a mix that can be store indefinitely and whipped up "in a flash" recipe.

WACKY CAKE
- Mix well and store:
1 1/2 cups flour
1 C sugar
3 tb cocoa powder
1 t salt
1 t baking powder
1 t baking
- To make cake:
1/3 c oil
1 t vanilla
1 tb white vinegar
1 C warm water
In ungreased 8x8-inch pan combine well flour, sugar, cocoa, baking soda, baking powder and salt. Make three holes in mixture; pour oil into one, vanilla into another and vinegar into the third. Add warm water and mix until well blended. Bake in preheated 350 degree oven 35 to 40 minutes.

Another similar type of recipe,but with a limited shelf life, is:
Single Serve Cake Mix
2 C Flour
1 1/2 C Sugar
3 1/2 Ts Baking powder
1 Ts Salt
1/2 C Butter
Combine flour and sugar, sift twice. Sift again and add baking powder and salt. Cut in butter. Store in covered container in Refrigerator 4 weeks or freeze 12 months.
To use:
Bring mix to room temperature. Combine 3 eggs, 1 1/2 ts vanilla and 1 c milk beat well and add all
of the mix. Beat 4 min. medium speed. Divide into 2 greased 9' layer pans. Bake at 350F for 30-35 min.

Other recipe collected but untried:

CAKE MIX plus recipe
6 c. flour
3 Tbsp. baking powder
1-1/2 c. shortening
4 c. sugar
2 tsp. salt
Combine flour, sugar, baking powder and salt. Rub in shortening until mixture resembles cornmeal. Store in airtight container in fridge
Plain Cake:
2 c. cake mix
1 tsp. vanilla
1 egg, slightly beaten
1/2 c. milk
Plain Cake: Stir ingredients for plain cake until well blended. Pour into 8-inch square pan. Bake at 350 degrees for 25 to 30 minutes.