Welcome to the Cake Decorators Q&A
Hi, can you use impression mat on the chocolate garnach or buttercream to give you a pattern.
There's no reason why it can't be done. You would need the consistency of the buttercream or ganache to be tacky rather than completely set. If you do it on a freshly covered cake, it will be difficult to form a sharp imprint.
I would suggest refrigerating buttercream covered cake until the consistency is firm to touch. Remove from the fridge and bring it to room temperature, then use the texture mat carefully to make the imprint. There are several tutorials on youtube which you may find useful. Search on youtube ' texture mats with buttercream'.
Ganache might be tricker but have a go anyway. Once the cake is covered, let the ganache set up to an almost firm but tacky consistency, then apply as with the buttercream technique.
Have fun experimenting with different types of texture mats. 🙂