I see a lot of posts about doctoring boxed cake mixes...replace water with milk, add an extra egg and replace oil with melted butter...my problem is that I’ve heard to double the amount of the oil (if 1/2 cup oil use 1 cup of melted butter) and I’ve heard using the same amount of melted butter for oil...  I’m not sure which one to do.  I think I tried this a few years ago and doubled the amount of butter and my cake sank in the middle!  What’s the best way to doctor a cake mix!  TIA

Have you seen kakeladi’s WASC recipe? It’s on here and a lot of people swear by it. You start with a mix and add other ingredients to it and it’s supposed to be amazing. I haven’t tried it yet but I think it’s the go to doctored cake mix recipe. 

If I use a box mix, I use milk instead of water, oil and eggs are the same, and I add a teaspoon of real vanilla extract. 

I almost 99.99% of the time, make my cakes from scratch. But when I doctor a mix, I just follow instructions from the particular brand, the usual water, eggs, oil etc. I personally never use melted butter, I find the cake too greasy. I always replace some of the water, 1/2 cup, with either yogurt or sour cream. And I always add extra extract/flavouring. I have a pink lemonade cake that although I can make from scratch, cake actually tastes better from a doctored yellow cake mix.

https://www.cakecentral.com/recipe/7445/the-original-wasc-cake-recipe     This should become your *one and only!* 'extended' box mix recipe!!    Don't even think of using anything else :) 

I use kakeladi's WASC recipe a lot because it makes a nice, sturdy cake for stacking and carving.  I can attest that it is delicious!