How can I create a custom button like this?

It should be just clickabl
save that as a png and and put it in your drawables folder. Then in your xml use something like this
I am not 100% positive that the corner cut out is going to work properly. That corner area that is gone may end up being clickable. If that is the case and if you don't want it to be then you'll have to slice your picture in half somewhere create two buttons that you can set next to each other to make up that shape and use the same click listener for both. From the users perspective it will still seem like one button.