Peel the shell, devein, and cut the back of shrimps so that it absorbs more flavors being marinated.
Clean shrimps with potato starch and rinse under cold running water.
Pat dry the shrimps with paper towel and place them in a medium bowl. Add all the ingredients for marinade into the bowl rub well with shrimps.
Cut white part of scallion into small pieces and keep it aside (use it later). Mince the ginger and garlic and put them together in a small bowl.
Heat oil on a large non-stick frying pan on medium high heat. When the pan is hot, cook shrimps until both sides change color (inside doesn’t have to be cooked through at this point). Turn off the heat and transfer the shrimp to a plate. Quickly rinse the pan.
In the same frying pan, heat oil over medium heat and sauté the white part of scallion, ginger, and garlic until fragrant. Lower the heat and add the chili bean paste and ketchup.
Then add chicken broth (water), sake, and sugar to the pan.
Continue cooking for a few minutes on simmer. Then return the shrimp to the pan and add the green part of scallion. Cook until shrimp is cooked through, stirring occasionally.
Add the sesame oil and then pour the potato starch/water mixture to thicken the sauce. Stir well and remove from heat. Transfer the shrimp to a serving dish.
You can keep the leftovers in an airtight container and store in the refrigerator for up to 3 days and in the freezer for a month.