If you're the only software developer in an early stage software company, one of two things is true:
- you have a substantial personal stake in the success of the company. For example, you may have stock options or restricted shares.
- you don't have such a stake.
The way you announce your decision to leave needs to be based on your stake in the company.
If you do have a big stake, and you've decided rationally that your new offer has greater potential than this company, that's a very consequential decision. You've decided that the present early stage company isn't worth any further investment of your time. That most likely has been a painful decision. When you announce it, your colleagues and possibly the investors may try to change your mind. You should be open to that.
You should also be prepared for them to be upset: you are likely to be the person confronting them with an unpleasant truth (their company isn't going anywhere).
If you don't have a big stake, get going! Tell your new place you need four weeks to wrap up your present work. Give the present place four weeks' notice, and do your best to finish your work and help hire someone to replace you.
In either case thank them profusely for the opportunity to work with them, and wish them the best.