How can I fix brownies that I accidentally added ketchup and pepper to?

Perhaps your brownies could be saved, and even improved, by the addition of a caramel layer on the top, creating a salted caramel and chocolate version. Simply boil sugar with a small amount of water until it's a deep golden amber color, take off the heat and whisk in several tablespoons of heavy cream and several of butter. Drizzle over the top, before or after cutting into squares, and let cool completely.

So I'm pretty sure that your brownies, as brownies, are un-salvageable. Tossing on more chocolate, or caramel, or whipped cream, is probably tossing good ingredients after bad - the addition of ketchup and pepper will not quietly fade away, and nothing will take them out.

That's just what the situation is, you may have been able to spice over some of the taste if you'd caught it pre-baking, but it would have likely taken a lot of diluting (extra flour and water and binders) and a lot of spicing (cinnamon and nutmeg and clove, cardamom, coffee, ginger, all of them, all of them) and then think about extra distracting additives like caramel or whipped cream to make it palatable - not great, just palatable with a niggling off taste at the core.

What you might be able to do, maybe, if you really have a reason to not want to waste the ingredients and are willing to put in the extra effort, is salvage the pan-full as an ingredient for further cooking. You would have to taste it carefully, to see whether the overall effect was salty or sweet. That is, it would still be both - but which category you put it in will depend on which is stronger to the tongue (you call it salty and creamy, but whether that's the overall flavor, or just the as-compared-to-other-brownies flavor is your call). You might dry it out into crumbs, and use it as an additive to larger dishes - well diluted, maybe using a cupful or maybe half that per recipe.

A cupful of these crumbs might be added to a heavily spiced cake for a bit of extra depth and just a touch of a savory peppery kick. A half a cupful might go in a hamburger mix, or a stew, for some extra depth and a bit of sweetness - depending on the specific results of your taste test, and your own food preferences, of course. Chocolate itself can go either sweet or savory (it gives depth, a touch of bitterness, and can work in a lot of dishes), and you have both sugar and fats, and salt and pepper in the mix - just use sparingly to add a touch, a touch of depth and complexity and balancing flavor to your recipes.

Really, I think it's worth tossing the whole thing, there isn't a lot you can do with it. Either the pepper or the ketchup may have been salvageable with a lot of work and tolerant tasters who are willing to re-set their expectations, but both is just too much. Using it as an ingredient in a larger dish (or three) might salvage the ingredients, but it's a lot of work and a single panful is often not worth the effort.

Continue adding spices, starting with cinnamon or clove. Eventually it would come to resemble that of a chocolate spice cake.