Cake mixes have stabilizers, emulsifiers and additives to provide leeway in terms of overmixing, undermixing and slight variances in oven temperature -- if you mix the right proportions of wet to dry ingredients. Cake mixes' forgiveness of slight errors even extend beyond inadequate or enthusiastic mixing, and can help overcome a loose, or runny, consistency -- if you know when you have modified the batter to the proper stiffness. Cake-mix batters should have a "dropping" consistency that results in a spoonful of batter dropping from a tilted spoon in 2 seconds.
One common-sense remedy for adding too much of a wet ingredient in a batter is compensating with more of a dry ingredient -- one that matches the texture of the dry ingredients in the mix. The closest you can come at home to creating a dry additive that matches the texture and sweetness of a box mix is with cake flour and confectioner's sugar. Combine 1/2 cup each cake flour and confectioner's sugar in a mixing bowl, using a whisk. Whisk the flour-sugar mixture into the cake batter, 1 tablespoon at a time, until the batter reaches a dropping consistency.
Cake mix ingredients are highly conditioned and ultra-processed for even dispersal -- that's why you don't need to sift them before mixing. So is instant pudding. Thickening a runny batter with instant pudding has two advantages: flavor matching and texture. No mixing, sifting or sweetening -- simply whisk in a matching flavor of instant pudding until the batter reaches a near-dropping consistency. There is one caveat: A little goes a long way. You want to reach just shy of a full dropping consistency, as the batter thickens more after the gelatin activates during baking. Whisk in instant pudding 1/2 tablespoon at a time until you can tilt a tablespoonful of batter and it slides off after 1 to 1 1/2 seconds, instead of the full 2 seconds.
Eggs thicken batters two ways: The protein in the albumen fortifies the cake's structure, giving it a firmer texture upon baking, and the lecithin in the yolks emulsifies the ingredients, creating tighter, stronger bonds between the fat, sugar and flour. The fat in the yolks also produces a moister crumb than that produced by box mix alone. Beat 6 egg whites and 3 egg yolks into 1 batch of cake-mix batter.
Dredge garnishes in cornstarch. Cornstarch has twice the thickening ability of flour, and if the batter is slightly loose, 1/2 cup of nuts or dried fruit per batch dusted in cornstarch tightens it. Add the dusted garnishes to batter after it is mixed, folding them in to disturb the batter as little as possible.