One technique that I use often to make a texture for miniature cake slices is the teasing technique with a pin or sharp metal skewer. I take the sharp pin and make swirling motions on the side of my cake, and this makes the crumbly texture. Poking holes in the clay won't work, only circular motions with the sharp pin will make it look good. Once I've done all sides, I bake the clay before decorating.
Here are some miniature cakes and tarts where I've used the teasing: