Line an 8×8-inch square baking pan with parchment paper, leaving overhang on two sides to create handles. Lightly spray the parchment with nonstick spray or rub with a small amount of butter to make removing and slicing the fudge easier.

In a medium saucepan, add the semi-sweet chocolate chips, sweetened condensed milk, unsalted butter, hot cocoa mix, and heavy cream (if using). Do not turn on the heat yet; allow all ingredients to be in the pan so they warm evenly.

Set the saucepan over low heat. Stir constantly with a silicone spatula or wooden spoon as the chocolate chips melt. Keep the heat low to prevent the chocolate from overheating, which can cause the fudge to become greasy or grainy. Continue stirring until the mixture is thick, glossy, and completely smooth.

Remove the pan from the heat once the mixture is smooth. Stir in the vanilla extract and fine sea salt until fully incorporated. Taste a small amount carefully; if it seems too sweet, add a tiny extra pinch of salt and stir again.

Let the fudge mixture cool for 2–3 minutes off the heat, stirring occasionally so it cools evenly. Gently fold in 1 cup of the mini marshmallows. They should soften slightly but mostly hold their shape to mimic marshmallows in hot cocoa.

Scrape the fudge mixture into the prepared pan. Use a spatula to spread it evenly into the corners and smooth the top. If the mixture is very thick, press it gently into an even layer with the back of the spatula or an offset spatula.

Immediately sprinkle the remaining 1/2 cup mini marshmallows evenly over the surface. If desired, add crushed candy canes or peppermint candies and mini chocolate chips or chocolate shavings. Gently press the toppings into the fudge so they adhere as it sets. For a salted hot cocoa variation, sprinkle a small pinch of flaky sea salt over the top.

Cover the pan lightly with foil or plastic wrap and refrigerate for at least 2 hours, or until the center of the fudge is firm to the touch. For the cleanest slices, chill 3–4 hours or overnight.

Use the parchment handles to lift the chilled fudge out of the pan and place it on a cutting board. With a sharp knife, cut into about 36 small 1-inch squares or 25 larger 1.5-inch squares. Wipe the knife with a warm, damp cloth between cuts for neat edges.

Serve the fudge slightly chilled or at cool room temperature. Store leftovers in an airtight container at room temperature for up to 4 days if your kitchen is cool, or in the refrigerator for up to 2 weeks. For longer storage, freeze for 2–3 months; thaw in the refrigerator, then bring to room temperature before serving.