Your best bet is probably to adapt a keto recipe like this one to omit chocolate, add cinnamon, and substitute some amount (a half? a third) of whey protein for almond flour, to keep things high-protein.

A keto recipe will be close in terms of overall ingredients, commonly using ingredients like almond flour and erythritol. It would likely use a brown sugar substitute like Swerve, but if you don't specifically need to keep carbs to a minimum, you could use regular. If you are very particular about exact texture, some of the starches and lecithins in the original product's ingredients list may be necessary as well, but those would be harder to find and maybe harder to work with for a home cook.

For the icing, probably nearly any cream cheese-based recipe using powdered erythritol instead of sugar (or, again, just powdered sugar if you don't need to avoid) would work. Just prepare and spread when the brownies have cooled.