Caramelize the Sugar: In a medium saucepan, heat the sugar over medium heat, stirring constantly until it melts and turns a deep amber color. Be careful not to burn it. Once caramelized, carefully add the hot water, little by little, stirring constantly. The mixture will bubble vigorously, so handle it with care. Continue to stir until all the caramel has dissolved in the water. Set aside to cool.

Prepare the Batter: In a large mixing bowl, whisk together the flour, baking soda, and salt. In a separate bowl, beat the eggs lightly and then add the condensed milk and melted butter. Mix until well combined.

Combine the Mixtures: Gradually add the egg mixture into the dry ingredients, stirring gently to combine. Once well incorporated, slowly pour in the cooled caramel syrup. Mix until you get a smooth and slightly runny batter.

Rest the Batter: Allow the batter to rest for about 15 minutes. This helps the baking soda to activate fully, which will create the signature honeycomb texture in the cake.

Preheat and Prepare the Pan: Preheat your oven to 350°F (175°C). Grease and line a 9-inch round cake pan with parchment paper or use a non-stick pan.

Bake the Cake: Pour the rested batter into the prepared pan. Bake in the preheated oven for 45-50 minutes, or until a skewer inserted into the center comes out clean. The cake should have a rich brown color with a honeycomb-like structure inside.

Cool and Serve: Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely. Once cooled, slice and serve. Enjoy your Kek Gula Hangus with a cup of tea or coffee.