In the bowl of a stand mixer, place 3 1/2 cups of the gfJules flour, the potato flakes, baking powder, sea salt, sugar, and yeast. Whisk to combine.
Add the room temperature kombucha, eggs, and butter. Engage the stand mixer and mix until all the ingredients are incorporated. Use a spatula to scrape the dough down into the bowl. Then turn the mixer on medium-low and allow the dough to mix (un-interrupted) for 5 minutes.
Once the dough has been mixed for 5 minutes, scrape down the sides of the bowl with a spatula and gently shape the dough into a mound in the bowl. Cover the bowl with a towel and place it in a warm area (70-75°F is ideal) to rise.
The dough should rise for up to 1 hour. While the dough is rising, line a 9x13" glass baking pan with parchment paper. The dough is done rising when it appears fluffy and has risen in the bowl, although it may not double in size.
After the rising time, add the remaining 3/4 cup of gfJules gluten-free baking flour and the 1/4 cup of King Arthur gluten-free flour. Mix the flour into the dough using a spatula. A smooth dough that holds together should form.
Turn the dough out onto a floured surface. At this point, use a spatula to manipulate the dough into a rough rectangle and divide the rectangle into 12 pieces, cutting through the dough with your spatula. Add more flour as necessary to keep the dough from sticking to the work surface.
Flour your hands and pick up each piece of dough to gently shape and roll it into a ball. Use a little warm water to patch up any cracks in the roll. Place each shaped roll on the parchment paper lined baking pan. If you want your rolls touching, place them about 1/2" apart on the pan.
Once all the rolls have been shaped and are on the pan, cover the pan with a towel or oiled plastic wrap and place the pan in a warm environment for the rolls to proof for 30-90 minutes. The proof time will depend on the warmth of environment. The rolls should rise some and become fluffy, but they should not overproof and develop air bubbles or pockets.
While the rolls are proofing, preheat the oven to 375°F. Once they have finished proofing, bake the rolls for 15-20 minutes. The rolls should become golden brown.
Remove the rolls from the oven and immediately cover them with a towel for 10 minutes. After 10 minutes, remove the towel and brush the rolls with melted butter if desired, or just leave them plain.
Serve the rolls warm for the fluffiest texture! Store leftover rolls in an airtight container or freeze them.