Prep the sweet potato: Wash the sweet potato, peel it, and cut it into 1cm cubes (smaller pieces steam faster). Place the cubes on a steamer rack, cover the pot, and steam over high heat for 15-20 minutes (It's done when a chopstick pierces the cubes easily with no hard center).

Make the mash: Transfer the steamed sweet potato cubes to a large bowl. While still hot, add the 15g of condensed milk. Use a fork or potato masher to mash repeatedly until the mixture is smooth and free of lumps (If the mash seems dry, add 5ml of milk to adjust the moisture, ensuring a smooth paste for easy layering). Let it cool to room temperature (to avoid melting the cheese slices later).

Preheat oven: Preheat the oven to 180°C (350°F), using both top and bottom heat, for about 10 minutes (The oven must be fully preheated for even cooking, preventing a raw center or sticky bottom). If your oven doesn't have a preheat function, set it to 180°C and let it run empty for 10 minutes before baking.

Make the batter: In a large, clean, grease-free bowl, add the 200g of glutinous rice flour and crack in the 1 egg. Add the 35g of fine sugar and 40g of corn oil. Mix for 30 seconds until the sugar begins to dissolve and the oil and egg are incorporated (No need to whip, just mix until combined).

Add milk: Slowly pour in the 185g of milk while stirring continuously. Mix until you have a smooth, lump-free batter with a consistency similar to pancake batter.

Strain batter (optional): For an ultra-smooth mochi texture, strain the mixed batter through a fine-mesh sieve to remove any unmixed flour lumps. This results in a more tender mochi (This step is optional and doesn't affect the overall flavor).

Prep the pan: Line the bottom and sides of a 6-inch pan with parchment paper (let the edges overhang by 2-3cm for easy removal later). If you don't have parchment paper, lightly grease the inner sides of the pan to prevent sticking.

Layering: Slowly pour half of the mochi batter into the prepared pan. Next, evenly spread the cooled sweet potato mash over the batter. Gently press it flat with a spatula (Be careful not to press too hard, to avoid mixing the layers). Then, lay the 3-4 cheese slices on top of the sweet potato layer (overlap them by 1/3 to ensure every bite has cheese and even meltiness). Finally, pour the remaining half of the mochi batter over the top. Use the spatula to smooth the surface. Gently tap the pan on the counter 2-3 times to release air bubbles (bubbles can create holes inside the mochi, affecting the texture).

Bake: Place the assembled mochi pan on the middle-lower rack of the preheated oven (if your oven has only one rack, use the middle position). Bake at 180°C (350°F) for 45 minutes (Oven temperatures vary; check the mochi's status after 30 minutes).

Check for doneness: The mochi is done when the surface is light golden yellow, the edges are slightly browned, and a skewer inserted into the center comes out clean (with maybe a few moist crumbs, but no wet batter). If the surface is browning too quickly, loosely cover it with foil after 25 minutes of baking to prevent burning before the inside is cooked.

Cool and unmold: Once baked, remove the mochi from the oven and let it rest on a cooling rack for 5-10 minutes (The mochi is very hot inside; unmolding immediately can cause it to break. Resting helps it set and firm up slightly). Once it has cooled slightly, lift the mochi out using the overhanging parchment paper. Place it on a plate and carefully remove the paper.

Slice and serve: Use a sharp knife to cut the mochi into 4-6 pieces (For cleaner cuts, dip the knife in hot water and wipe it dry before each slice). Enjoy best while warm. The cheese will be perfectly molten and stretchy. If it cools down and the cheese solidifies, reheat it in the microwave on high for 30 seconds to restore the molten texture.