How to turn a cookie recipe into a cake recipe

Sorry this is 7 years later -

I finally did this with my cookie recipe that I love and got it right after a few attempts.

1) Substitute part of the butter/shortening with liquid oil. My recipe calls for 1 cup butter and 1/2 cup shortening - so I made my cake with 1/2 butter and 1 cup canola oil. Any version of 1.5 cups of fats would have probably been OK. I could have done 3/4 cup of each or 1 cup butter to 1/2 cup oil (I'll think I'll try this next time to preserve butter flavor, and I think I'll melt and brown the butter).

2) Add liquids. I added 1 cup applesauce, some maple syrup, and extra vanilla extract. I also planned on adding a cup of milk, but since my cookies have 1 cup of oats, I let the oats soak in the milk overnight in the fridge then added the "overnight oats" to my cake batter.

3) Add baking powder. Cookie recipes usually have baking soda - cake recipes usually have the same amount of baking soda with a little bit of baking powder - usually about 2 tsp soda and 1/2 tsp powder. If you are at a high altitude, you might have to reduce some of this so research high altitude baking.

At this point, with everything else the same - eggs, sugar, salt, flour, spices - my batter was kind of runny. So I added a little more flour. But once the cake came out it was a little bit dense, so I'm thinking I could have just stuck to the original flour.

Obviously, measurements are going to be a guess. But you're going to get cake that tastes like your favorite cookies, and you'll get a new "family" recipe to share.

Recap:

1) Substitute oil for some of the solid fats
2) add some liquids like syrups, milk, cream, fruit purees
3) add baking powder along with the baking soda
4) add more flour (or don't) to get the batter to what you think would be a good density

Couldn't edit my question, so have to add a bit here. The primary ingredients, the biggest bulk, is 2 cups brown sugar, and 4 cups flour. Given it makes great cookies, having swapped out every baking spice I could so far, figured I'd try it as a cookie-cake kind of thing.

I've made cookies into cake-like bars by beating a couple of eggs into the sugar & butter before I add the rest of the ingredients. Hope that helps.

I wouldn't use a cookie recipe for a cake, unless you're looking to make a cookie cake. For instance, a large chocolate chip cookie pressed into a pie plate

Can you share the recipe? What is it about the cookies that you like so much that you want them to be cake? Cakes generally have more liquid, eggs, and leavening than cookies. You beat the liquid ingredients longer to incorporate air so that the cake rises. If there's a specific flavor profile you're trying to create, we could probably help direct you to a recipe that could be modified. But if you really want to understand the science, and make it yourself, check out Michael Ruhlman's Ratio and/ or Shirley Corriher's Bakewise.
There are lots of other books on the science of baking, but those are the first two that came to mind. Good luck!