To make the dry mix: Combine all dry ingredients in a mixing bowl and whisk thoroughly. Pour into storage jars or containers. Label with contents and date. Store in a cool, dry place.
When ready to bake: Preheat oven to 350°F. In a large bowl, mix the eggs, vanilla, and oil into the dry mix until smooth. Add the milk and stir until fully combined with the batter. Scoop batter into 24 cupcake liners or pour into a greased 9x13 cake pan. Bake at 350°F for 20 minutes for cupcakes or 25 minutes for the cake. Test with a toothpick—it should come out clean when done.
Notes
Dry mix stores 3-4 months in airtight container. Label jars with wet ingredients needed. Use Dutch-processed cocoa for richer chocolate flavor. For vanilla cake, omit cocoa and use 3¼ cups flour.