May I use cacao butter in place of coconut butter?
Not knowing your recipe (but I did some general googling re. fat bombs in general), I'd say go for it.

The main differences are based on the higher melting point for cocoa butter, which can actually make storage easier. At the same temperature, the cocoa butter based treats will be firmer - again a matter of taste. 

But: The coconut oil based fat bombs will probably give you a "cooling" mouthfeel, caused by a combination of high enthalpy of fusion and a melting point below body temperature, it's the key ingredient in "ice chocolate". Cocoa butter with a melting point around 30-38C / 86-100F will not do that.

The taste will differ a bit as both fats have their specific taste, in combination with other ingredients neither should be overpowering, though.