Can I use a dark cocoa candy bar in place of bakers chocolate?
I would say yes. However, depending on what kind of squares it calls for it may change the taste of the brownie or how rich the chocolate flavor is. 70% cocoa bars would be more similar to what Baker's Chocolate sells as Bittersweet (67% cacao) so if the recipe is calling for Unsweetened squares (100% cacao) then you'll have less chocolate flavor (and perhaps a slightly sweeter brownie). If the recipe is calling for Semi-Sweet squares (54% cacao) then you'll have more chocolate flavor (and perhaps a slightly less sweet brownie).