What am I doing wrong?

For the best results i'd use a gram or more.
Also tin foil and an empty stomach help.
Add a bit of vegetable oil to the Nutella for added absorption of THC.

 

Like TheJoy said, try adding more oil.

Also, if you're intent on sticking with the firecrackers rather than taking it further and making a proper oil, you can make your 'cracker less of a 'worst-case-scenario' edible by heating longer, and allowing it to sit for a day or two after heating to promote bioavailability. You should also decarb your herb, first.

Processing is crucial. It makes the difference between an intense 15-30 minute cerebral onset with long lasting, narcotic effects, and using more material just to wait two or three hours for it to kick in, with less of an overall buzz. With a properly made oil, you should need much less than you would normally smoke in a single session, to achieve a much stronger effect. The more corners you cut during processing, the more material you're going to need.

For instance, an average dose for a properly made oil is 0.35g, and most folks who either don't smoke every day or who are new to edibles, need to start with half a dose that size. A firecracker on the other hand, needs at least .5 - 1g, even for patients with a lower tolerance.

Due to their reduced processing and lack of bioavailability, firecrackers often don't work at all for people who are regular or heavy (multiple times per day, each day) tokers, whose bodies are accustom to regular, and more rapid cannabinoid delivery. A firecracker simply can't keep up with their systems demands, it can't flood the bloodstream with cannabinoids nearly as effectively as a well made oil, almost regardless how much canna the cracker has been loaded with.

Hope this helps

 

Bake at lower temp ~275 should do it