Step 1: In a medium saucepan, whisk together the unsweetened cocoa powder, granulated sugar, and salt. Ensure there are no lumps.
Step 2: Pour in about 1/2 cup of the milk into the dry ingredients. Whisk vigorously until a smooth, thick paste forms.
Step 3: Gradually add the remaining 3.5 cups of milk to the saucepan, continuing to whisk until everything is fully combined.
Step 4: Place the saucepan over medium heat. Heat the mixture, stirring frequently, until it is hot and just begins to simmer around the edges. Do not boil. Let it gently simmer for 2-3 minutes to deepen the flavors.
Step 5: Remove the saucepan from the heat. Stir in the semi-sweet chocolate chips (or chopped chocolate) and vanilla extract. Stir continuously until the chocolate is completely melted and the hot chocolate is smooth and glossy.
Step 6: Taste and adjust for sweetness if desired. Add a little more sugar if you prefer a sweeter drink, whisking until dissolved.
Step 7: Pour your creamy homemade hot chocolate into mugs. Serve immediately with your favorite toppings like whipped cream, marshmallows, or chocolate shavings.