To make the puffs:

1) Preheat oven to 425 F. Line a sheet pan with bakers parchment if using. In a small bowl, mix flour, salt, xanthan gum and sugar and stir to thoroughly blend.

2) Place the electric mixer with attachments near your stove. You'll be beating dough in the saucepan over heat. Place the water and butter into a small saucepan over high heat. Once butter is melted, reduce heat to a moderate bubble for 1 minute. Insert mixer blades into water/butter and start mixer on lowest speed. Add the flour all at once and increase speed to medium low. After 10 seconds remove from heat and continue beating until dough looks like loose oatmeal.

3) Turn dough out into stoneware bowl. If you have an instant-read thermometer, monitor dough temperature as you use a spatula or spoon to move the dough around, pressing it against sides of bowl to aid cooling. You need to reduce heat to around 120 F to prevent eggs from cooking as they are added. In my 74 F kitchen, using a stoneware bowl that's 1/2 inch thick,  this took 2 minutes 10 seconds.

4) Add the first egg to the dough and beat on medium. The dough will go furry, then sticky, then glossy but will likely not pull together into a cohesive mass. Continue to beat for 1 minute after the glossy phase then stop mixer and add the second egg. The same appearance changes will occur, but after about 20 seconds the dough will suddenly cohere around a mixer beater. Continue beating until dough releases (hopefully not on your shirt!). Scrape from beaters.

5) Push dough into the scoop then release hemispheres onto parchment. To get smooth results you may need to wipe out the scoop with a wet fingertip after each hemisphere. Use a fingertip or small spatula to smooth the balls. 

6) Bake 10 minutes at 425 then reduce heat to 350 and bake another 15 - 25 minutes, or until "puffs" are medium-tan. Remove to a cooling rack. They will likely collapse but that's OK - we're going to re-inflate them with pastry cream!

To make the pastry cream and fill the puffs:

1) Measure cream and milk into a calibrated cup and stir to combine. Place sugar and cornstarch  into separate small bowls. Separate eggs, placing yolks into a small bowl. Place the ice in a large bowl. Place the saucepan atop the ice and add enough cold water to the bowl to reach 2/3 up the outside of the saucepan. Remove pan from ice water bath and set ice water bath aside.

2) Add 1/4 cup of the milk/cream mixture to the cornstarch while whisking. Continue to whisk until a smooth paste forms - about 15 seconds. Place the remainder of the milk/cream mix into the saucepan on medium heat.

3) While milk/cream mix warms to a light bubbling simmer, add the egg yolks to the sugar and whisk until smooth. Remove milk/cream mix from heat. Withdraw 1/8 cup of the hot liquid and add slowly to eggs, while whisking continuously. Return saucepan to heat and raise heat to medium-high, until mixture bubbles. Add the tempered eggs, along with the cornstarch paste, to the milk/cream mix, whisking lightly but continuously until fresh bubbles begin to arise. Mixture should have thickened noticeably. Remove from heat.

4) Place saucepan into ice water bath. While whisking, add the vanilla and the butter. Stir until the butter has melted and all ingredients are blended.

5) Pour the pastry cream into a medium bowl. Cover with plastic wrap, gently pushing the wrap down onto the surface of the cream. Refrigerate at least 2 hours.

6) Pipe cream into puffs or split puffs and use a butter knife to smear generous portions into puffs. Assemble puffs.

To add chocolate glaze:

Microwave 1/3 to 1/2 cup chocolate chips and allow to cool just slightly before pouring or brushing over the puffs.