Put the pitted cherries into the inner pot of the pressure cooker. Add the sugars, cinnamon, water and cherry flavoring. Stir to combine. Put the pressure lid on and turn the valve to seal. Pressure cook for 5 minutes on high. When the time is up, immediately release the pressure.
While the cherry filling is cooking, make the cake batter. In the bowl of a stand mixer or a large mixing bowl, combine the flour, sugar, baking powder, and salt. Stir to mix together.
Add the butter in Tablespoon pieces and blend together with either a fork or a a potato masher until it resembles sand. This can also be done in the stand mixer with the paddle attachment on low or with a handheld electric mixer.
Mix the buttermilk, egg, and vanilla extract together and pour in about ⅓ of it into the dry mixture. Fold to incorporate. Add another ⅓ of the wet mixture and fold into the dry. You don't want to overmix. If using a stand mixure, use the low speed. Add in the final ⅓ of the wet mixture and fold just until it's incorporated. A few lumps is fine. The consistency should be like a cake batter.
When the cherry filling is done pressure cooking, mix up the cornstarch slurry. Combine the 2 Tablespoons of cornstarch with 3 Tablespoons of water and stir until it is a smooth consistency. Turn the sear/sauté on high and bring the cherry mixture to a boil. Pour in the cornstarch slurry and stir until thickened.
Decrease the sear/sauté to medium. The mixture should still be boiling, but not a hard boil. Spoon 2 Tablespoons of the cake batter on top of the cherry mixture at a time. Spread out the topping so there are some areas that aren't covered. When all the cake batter is on the top of the cherry mixture, sprinkle the demerara sugar on top if you are using it.
Close the crisping lid and turn the bake/roast to 325°F/160°C and bake for 10 minutes. Let the cobbler rest after baking and it's best served slightly warm or room temp. Enjoy!