Biscuits: Combine crushed crackers and buttermilk in bowl. Refrigerate 20 min. Meanwhile, combine flours, sugar, baking powder and salt in separate bowl. Cut in butter until mixture is the size of peas. Stir in buttermilk mixture until "shaggy" dough forms but not thoroughly combined. Place on parchment paper-lined sheet pan and pat down to 1-inch thickness. Refrigerate 30 to 60 min or until firm.
Remove dough from refrigerator and place on lightly floured surface. Roll out to 1/2-inch thickness. Fold into thirds. Turn dough a half turn and repeat rolling and folding once. Refrigerate 30 min. Remove dough from refrigerator and roll on lightly floured surface to 3/4-inch thickness. Use 2-1/2-inch round cutter to cut out 15 biscuits. Transfer to parchment paper-lined sheet pan.
Place biscuits in 400°F convection oven, then immediately reduce temperature to 375°F. Bake 12 to 15 min. or until golden brown. Cool completely in pan on wire rack. Cover and store at room temperature until ready to use.
Breading: Combine flour and potato starch in bowl. Remove 5 oz. flour mixture to second bowl; add water and salt. Stir to form batter. Combine cracker crumbs and bread crumbs in third bowl. Coat each chicken thigh with flour mixture, dip in batter, then coat with crumb mixture. Refrigerate until ready to use.
Gochujang Glaze: Combine all ingredients except scallions in saucepan. Bring to boil on medium-high heat, stirring frequently. Boil 1 min., stirring constantly to prevent burning. Remove from heat; stir in scallions. Cool to room temperature. Refrigerate until ready to use.
For each sandwich: Cook 1 breaded chicken thigh in 350°F deep fryer 4 to 5 min. or until done (170°F). Split 1 Biscuit in half. Fill with chicken, 1 lettuce leaf, 1 oz. kimchi, 1 oz. warmed Gochujang Glaze and about 1 Tbsp. scallions.