Baker's chocolate is essentially cocoa powder and cocoa butter. Usually a little lecithin is added to make it smooth and sometimes it has some sugar.
Substitution tables suggest that the unsweetened chocolate can be replaced with 3 parts cocoa and one part butter.
This means that one cup of melted baker's chocolate could be used in place of the 3/4 cup of cocoa and 1/4 cup of the fat in the cake recipe. Obviously it would have to be added with the wet ingredients instead of the dry as the cocoa is.
If the baker's chocolate is bittersweet or semisweet instead of unsweetened then the sugar in the cake would need to be reduced accordingly.
The lecithin in the baking chocolate will actually improve the consistency of the cake.