I want to substitute liquid oil with a dry product

Bisquick is a dry product that contains partially hydrogenated soybean and/or cottonseed oil and leavening. You can get recipe ideas from their website. With the Bisquick, some powdered eggs and powdered milk, you could create all kinds of fun mixes for camping.

I just take a little vegetable oil in a small Nalgene bottle that seals tight. For a week with 9 people I wouldn't even use up 200ml, and I presume you're doing a shorter trip with less people, so the weight is not an issue. You can then add it to what you're cooking, use it to fry things, etc. (I also take cornmeal to fry pizza or English muffins.) Using regular oil and regular recipes makes a lot more sense than trying to use something unusual.

You can consider using lard, shortening, or ghee. They are not dry, but they are more easy to store and carry and melt less easily than butter.

In case fat content is a concern, here is a quick list:

Vegetable oils have about 15 gms of saturated fat per 100gms of oil.

Vegetable shortening has about 20 gms of saturated fat per 100gms of shortening.

Lard has about 40 gms of saturated fat per 100gms of lard.

Butter has about 50 gms of saturated fat per 100gms of butter.

Ghee has about 60 gms of saturated fat per 100gms of ghee.

Shortening, lard, and ghee all respond differently to heat, so it helps if you're familiar with how they respond when you're using them to cook. Ghee also has a distinct aroma, so if you or those who would eat the food are not familiar with ghee, you should try it out first before deciding on it.

In addition to this, I am trying to work through your specific use-case. You can still actually use oil or butter. The advantage is that fats don't spoil easily. I'm not sure what or how you intend to cook, but you can consider the following techniques:

If you have chunks of food, like chopped vegetables, pieces of chicken, etc. that you are sure to cook, you can just coat the food in butter, salt, and seasoning, which you can then wrap in foil and store in a container. It will be easy enough to carry without spillage. When you need to cook it, just toss them into the cooking vessel, and cook till they slightly sear. Then, you can add water and let it simmer till done.

If you have things like ground beef or similar food, again you can put butter, salt, and seasoning in the mixture, and make them into small dollops (like meatballs), and you're good to go.

If you can provide more details about what kind of dry foods you are trying to pack, then I am glad to suggest other (possibly more suitable) solutions.

there is a powdered cocoa butter product called mycryo. it's intended for frying, but it might work for your intended purposes. http://www.cacao-barry.com/usen/2516

There is a thing called powdered shortening or you can get powdered butter. They are just add water products that you can put in a recipe like that and it will work fine.

I agree with just making the brownies ahead of time and putting them in zip lock bags.

If you really want something you can make during camping, you could try the 3-2-1 cake recipe. Take a box of angel food cake and mix it with a box of another cake mix (chocolate, vanilla, spice, lemon, etc.). Then take 3 TBSP of the mixture, mix with 2 TBSP liquid (can be anything), and microwave for 1 minute. I'm sure you can adjust it to cook over a fire somehow. I've made this using chocolate cake. To me it still tastes like angel food cake and not very chocolatey. Maybe using any other flavor would be good. You could try it with a brownie mixture. I might work. It will won't give you the brownie texture (it is airy like angel food cake), but it might impart some brownie qualities.

You could make your own powder oil by adding some maltodextrin to the oil you want to powderize. About 4 gram of maltodextrin to 10 grams of oil.