To make the teriyaki sauce, combine the soy sauce, brown sugar, garlic powder, ginger powder, and mirin in a saucepan and mix well. In a small bowl, mix the cornstarch and water to make a slurry and add it into the soy sauce mixture. Cook the sauce over medium heat while stirring occasionally until thicker in consistency.If the sauce becomes too thick, add some hot water. Remove from heat and sauce aside.
To make the baked crispy chicken, pound the chicken using a meat pounder until it is uniform in thickness. Season both sides with salt and pepper. Coat chicken with all-purpose flour, then dip into beaten egg. Generously coat the chicken with Panko breadcrumbs. Line a baking sheet with parchment paper and bake chicken in the oven at 380 degrees Fahrenheit for about 35 minutes, until crispy on the outside and tender and thoroughly cooked on the inside.
Heat cooking pan on medium high heat and melt 2 teaspoons of butter. When the butter is completely melted and the pan is hot, grill both sides of each bread slice until golden brown. To assemble the sandwich, place the lettuce leaf, tomatoes, and cucumbers on a slice of toasted bread. Place the baked crispy chicken on top of the bed of vegetables, and pour Teriyaki sauce on top of the chicken. Close the sandwich with another slice of bread. The amount of Teriyaki sauce to put in varies according to your preferences.