With the current version of the app (v 2.2.3) it's a little tricky to do this.
In the new version (v 3.0, scheduled to be released in June 2017) it will literally take 10 seconds to do it. Maybe 5 if you are fast. I will send you a video on it how to do it.
Here are the steps currently (v 2.2.3):
- create the original cube
- draw triangles on the sides at the corner
- create a new cube
- align it on a triangle baseline (with translate tool)
- rotate the new cube with 54.736 degrees
- subtract the cubes
And you are ready.
Here's a short video explaining the steps.
In the new version, this will be much easier (chained fillet + delete). Max 10 secs.