Chocolate – What can be substituted for cocoa nibs

You can use compound chocolate (the inexpensive "coating chocolate"). It's what you find used in a lot of cheap store-bought chocolate, and it has very similar melting and viscosity properties to couverture.

You can usually find it in most bulk food stores.

The important property of couverture is its high proportion of cocoa butter and correspondingly high fat content. Compound chocolate normally does not contain any cocoa butter, so it is nowhere near as rich as couverture, but it achieves roughly the same fat content with vegetable fat and will act similar to couverture when melted.

Technically, compound chocolate is not actually chocolate, so the result will be lower quality than if you had used couverture; but if you're pinching pennies, I think that's the way to go.

You can't add too much chocolate into a cake, as you'll end up adding too much fat, which will change how the cake sets up. They likely wanted more chocolate flavor than what they felt they could get from chocolate alone, and so opted to add cocoa powder as well.