Meat – use live yoghurt to kick-start fermentation

I guess it depends on what the substitution is for.

Certainly if the butter is just for flavour, it's a reasonable substitute (I think I'd use slightly more yogurt). But 9 times out of 10, fat is the main reason the recipe is calling for butter! Yogurt cannot substitute for a fat (butter) because it has very little fat.

If you lower the fat content of a baked good then you'll generally end up with a product much denser and less flakey. So if you feel you must do this for some reason, then I would increase the sugar in the recipe to compensate (sugar inhibits gluten formation and will help to make the result less chewy).

The other thing you'd better keep in mind is that yogurt is naturally sour and will become even sourer when baked at a high temperature. The longer you bake, the more sour it will become, so you might need to amp up the sugar significantly to maintain the proper flavour.

So for the most part, no, yogurt is really not a substitute for butter or any other fat. Obviously this is partly a matter of personal taste, and if you're happy with the result, then continue doing what you're doing - but I think I'd definitely hesitate to eat a cake that was baked with yogurt instead of butter.

"Instant" coffee typically is disolved in hot water, so I'd think that just stirring it into a cold product would be less than ideal.

You might try making a coffee syrup (as you're adding sugar anyway) or try steeping the instant coffee in hot milk or water first, cool it down, then mixing that into the yoghurt.