I would do as fred_dot_u initially suggested, by increasing the bed temp (or using a hair dryer) to heat the BuildTak. Then, use a small fan to quickly cool the platform (or at least quicker than room temperature). An ice pack on the build plate/part could also work. This drastic fluctuation between the build platform (or BuildTak) and the part should make it easier to remove the part.
This works because the temperature coefficient is going to be different between the build plate, BuildTak, and the 3D printed part. So, each part rises and falls in temperature at different rates. When objects are heated and cooled, they expand and contract (respectively). Essentially, as each object expands and contracts at different rates, the objects begin to separate.
A good example is if you've ever placed a jar into the fridge/freezer to make it easier to open. Typically the jar is glass and the lid is either plastic or metal. You'll notice that the lid is significantly easier to open, as opposed to its original state, because the lid and the jar physically react differently to temperature changes.
Also, here is a good article explaining a few different ways to remove a stuck part. (for prosperity sake, here's a quick list):
- Brute force. Just try to yank on it until it pries off.
- Sharp objects. Try using a small blade to get under a corner of the part and wiggle the part a bit. Careful not to break the blade and send it flying.
- Utilize temperature difference. I already explained this above.
- Use solvents. I didn't know this, but apparently there are solvents in the market that are targeted for 3D printing maintenance. Essentially its just an alcohol-based liquid...
- Use floss. Another cool idea that's related to using a sharp object, using dental floss. Basically, any small object that you can remotely wiggle under the part and give you more leverage to yank on it.
- Invest in a flexible build platform. I've personally heard some mixed reviews on these (in present day of 2016). But material science is getting better every year, so who knows what will be available soon.