Remember that what makes a brownie a brownie is that its constituents are arranged and put together to make a brownie. Eating a brownie changes nothing other than separating its material and adding some of them to your body cells, unchanged or changed.
Similar things happen all the time. When breathing air, much of the molecular oxygen (O₂) you breath is dissociated into 2 oxygen atoms to be incorporated with other atoms. It’s the same thing as eating, digesting and, therefore, dissociating brownies except for that brownies are large enough in size to be seen by our naked eyes.
It is natural and happens all the time.
Also, remember that brownies are not living beings; they have no feelings. They never complained, and will never do.
I suppose that another reason for why you feel this way is because autistics like routines and hate changes like changing a brownie into small pieces or even eat it. Couldn’t it be that you feel this way because eating a brownie makes an actual change (although, subtle) in the world after eating it? The brownie is no longer there after you ate it; that’s the difference. And we don’t like changes.
If so, remind yourself that you ate brownies before and nothing weird happened except for that it tasted good!
When having such thoughts, always remind yourself that, although the situations of eating or "hurting" a brownie and actually hurting a living being are somewhat similar (which is probably why you empathize with brownies,) brownies do not feel anything, so you don’t actually hurt it. Why? Because it doesn’t have a nervous system; it doesn’t feel pain; it doesn’t respond saying “ouch!”
I know for sure that you know all of that already, but what works for me is always reminding myself of the same logical reason why it’s different, so this same idea "stucks" in my brain and is retrieved automatically whenever I have thoughts like those. That’s my point.
Hope it helps!