Hello, everyone!
I am going to be making this cake (the recipe for which I got of off CC a couple of years ago, I think):
Cinnamon Vanilla Cake
2 sticks butter (the real thing)
2 cups granulated sugar
4 large eggs
3 cups sifted all-purpose flour
1 ½ teaspoons ground cinnamon
½ teaspoon salt
1 cup whole milk
1 ½ teaspoons vanilla
Directions:
1. Preheat oven to 350˚ F
2. Cream butter until smooth and light. Add sugar and continue to mix until fully incorporated and fluffy.
3. Add 1 egg at a time, beating well after each addition. In a medium bowl sift together flour, baking powder, cinnamon, and salt.
4. Combine milk and vanilla.
5. Alternately add flour mixture and milk, beginning and ending with flour. Pour evenly into 2 greased and floured 9-inch cake pans.
6. Bake for 40 minutes or until golden cake should spring back to the touch.
However, I enjoy adding pudding mix and sour cream to my cake mixes (following the Durable Cake recipes here on CC) and when I made this cinnamon vanilla cake last time it was a little dry.
Can I just add a small box of vanilla pudding mix and a cup of sour cream to this recipe, do you think? Or would I need to change around other things as well?
I have learned from experience that adding pudding mix to a cake recipe results in a gummy cake.
Furthermore, adding sour cream will alter the acidity of the cake, which may result in the cake not rising properly.
This cake recipe follows the standard 1-2-3-4 formula for butter cakes (1 cup butter, 2 cups sugar, 3 cups flour, 4 lg eggs), which, when made properly, yields a cake that is moist but not soggy or oily and with a tight crumb.
My guess would be that you are improperly measuring the flour, resulting in too much flour in the batter, which will throw off the balance of the cake, resulting in it being dry.
This recipe states "3 cups sifted all-purpose flour", which means that you are to first sift the flour, then take 3 cups of the sifted flour. If you measure out 3 cups, then sift, you will have too much flour. Never scoop the flour into the measuring cup with the measuring cup itself. Instead, gently spoon the flour into the measuring cup, do not compact the flour, and then level off with the back of a butter knife.
Thank you ~ when I have sifted flour before I DID measure first and then sift. I didn't know any different, lol ... but now I do! It makes so much sense, now! 
As for adding pudding and sour cream, I do that with doctored cake mixes using recipes I found here at CC. I don't find them gummy, I like them - but to each her own, right? I have never, ever played around with a scratch recipe before, which is why I sought the advice of you experts here on the forums! Thank you so much for your help!
Blessings!
Agree with everything prterrell said. 
But since you asked, here are some scratch recipes which also include instant pudding mix as an ingredient:
http://cakecentral.com/cake-decorating-ftopict-651577-.html
HTH
Yes, but adding pudding or sour cream to a cake mix is NOT the same as adding to a scratch recipe. Cake mixes contain all kinds of emulsifiers and other things that make them very tolerant of alterations. Scratch recipes are not like that. Scratch recipes are chemical formulas. Alter the formula and you throw the balance of the cake off.
Remember, if it says "sifted flour" sift, then measure, if it says "flour, sifted" measure, then sift.
Yes, but adding pudding or sour cream to a cake mix is NOT the same as adding to a scratch recipe. Cake mixes contain all kinds of emulsifiers and other things that make them very tolerant of alterations. Scratch recipes are not like that. Scratch recipes are chemical formulas. Alter the formula and you throw the balance of the cake off.
Again, I totally agree.
Just thought OP might be interested in making a scratch cake (which includes instant pudding and sour cream as ingredients).
I just think it's always good to have more options. 
Quote by @%username% on %date%
%body%
