Line an 8x8 pan with parchment or waxed paper and lightly grease the paper.
Attach the whisk attachment to a stand mixer. Pour half of the water into the bowl and sprinkle with the gelatin. Let stand while preparing the syrup.
In a medium saucepan over medium heat, combine the remaining water, the sweeteners, the cream of tartar, and the salt. Bring to a boil, stirring to dissolve the sweeteners.
Using a candy thermometer or an instant read thermometer, bring the mixture to 237F to 240F. Remove from heat.
Turn the stand mixer on low and slowly pour the hot syrup down the side of the bowl. Once all of the syrup is mixed in, add the extract. Turn the stand mixer to medium high and beat until the mixture is lukewarm, thickened and white. This can take 5 to 15 minutes.
Working quickly, pour the mixture into the prepared pan and smooth the top. Let set for 4 to 6 hours, until the top is no longer tacky to the touch.
Flip out onto a cutting board and cut to the desired size. Dust with powdered sweetener, if desired. Let sit for a day exposed to the air to dry out a bit, then store in a covered container.
You really need the combination of two sweeteners here. One should be erythritol based, like Swerve or Lakanto. The other should be one of three possibilities: Bocha Sweet, Xylitol, or allulose.