In a small pot, add brown sugar and 1/2 cup of water. Bring to a boil using high heat. Continuously stir until all the brown sugar has melted. Turn down heat to low and continue to stir until syrup thickens. It will take about 3 to 5 minutes. Turn off heat and let sugar water mixture cool to room temperature.
While waiting for syrup to cool, prepare hard boiled eggs and vegetables. Peel and cut potatoes in half. soak potatoes in water for 10-15 minutes. Bring a pot of water to a boil and add in potatoes. Boil potatoes for about 15 minutes or until tender when poked with a fork. Remove from water and set aside. Once eggs are hard boiled, peel and cut into bit sized pieces. Set aside.
Cut cabbage into bite sized pieces. Boil another pot of water and add a pinch of salt. Once water is boiled, add in cabbage. Boil for 3 minutes and remove cabbage from hot water. Using the same boiling water, add in bean sprouts for 30 seconds then remove from hot water. Set aside.
Cut boiled potatoes into bite sized pieces. Wash and cut cucumbers and lettuce into bite sized pieces. Cut tofu into bite sized pieces. You can use store bought fried tofu and reheat in the air fryer or serve cold. You can also use regular firm tofu and remove excess liquid with paper towels. Set aside.
Put entire jar of peanut butter into a large bowl. Add in cooled syrup (about 1 cup), 1 teaspoon sriracha hot sauce, 2 teaspoons kecap manis, 3 tsp lemon juice, 1/4 cup warm water, and 1 tablespoon fried shallots. Mix until gado gado sauce is smooth and uniformed. Gado gado sauce will seem thick but will thin out once mixed with vegetables.
To serve, place all vegetables and toppings into a large bowl and add a few tablespoons of gado gado sauce. Mix everything together. Serve with extra fried shallots and shrimp crackers.
Store gado gado sauce in an airtight container in the fridge for up to 1 month. If serving gado gado sauce out of the fridge, you can place a few tablespoons into a microwave safe dish and microwave gado gado sauce for 10 seconds at a time until soft.