After playing through one of the three chapters, you should have access to the kitchen. Open up the fridge and pick up the Dope Cow Milk.
Go over to the microwave, and there should be a set of drawers to your left. Open the bottom drawer and pick up the Chocolate Powder.
On top of the counter, next to the microwave, there should be a plate/tray with a cup on it. Use both the Dope Cow Milk and the Chocolate Powder with the plate/tray.
Next, click on it to start a small cutscene where you pour in the Chocolate Powder and Dope Cow Milk into the mug, then place the mug into the microwave.
After the cutscene is done, go out of the kitchen towards the entrance hallway, then go up the stairs. Turn left, and enter the son's room on the right side, which should now be lit in a blue light. Walk straight ahead towards the spiraling, black circle in the opposite end of the room, and a new cutscene will start.
Enjoy your hot chocolate!