Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Beat brown sugar and butter together in a bowl using an electric mixer until light and fluffy. Add egg, honey, and vanilla extract and mix until smooth.
Combine gluten-free all-purpose flour, protein powder, coconut flour, baking powder, and salt together in a bowl. Slowly mix flour mixture into creamed butter mixture; fold in oats, rice cereal, chocolate chips, raisins, almonds, and flax seeds. Press mixture into the prepared baking dish.
Bake in the preheated oven until lightly browned, 20 to 30 minutes. Cool on a wire rack.
Be creative and adjust to your taste. Add or omit fruit, nuts, seeds, chips, and spices. Dried cranberries or cherries can replace the raisins. Sunflower seeds or pecans can replace the almonds. Use white, dark, or semi-sweet chocolate chips.