Step 1: Prepare Your Pan and Ingredients
Line an 8×8-inch (20×20 cm) baking pan with parchment paper or foil, leaving overhang for easy lifting. Lightly butter or spray to prevent sticking.
Measure all ingredients ahead of time—once you start, things move quickly.

Step 2: Combine and Heat the Mixture
In a heavy-bottomed saucepan, combine granulated sugar, brown sugar, butter, milk, cream, and salt. Heat over medium, stirring until butter melts and sugars dissolve.
Stop stirring and let the mixture boil gently. Attach a candy thermometer and cook until it reaches 234°F–238°F (112°C–114°C) — the soft-ball stage.

Step 3: Cool and Add the Chocolate
Remove from heat and let cool undisturbed for about 10–15 minutes, until the temperature drops to around 110°F (43°C).
Stir in the chocolate chips, marshmallow crème, and vanilla until smooth. Fold in any nuts or espresso powder if desired.

Step 4: Pour and Set the Fudge
Pour the mixture quickly into your prepared pan. Smooth the top and sprinkle sea salt flakes if desired.
Let it cool at room temperature for 3–4 hours or until firm. Avoid refrigeration during setting.
Once set, lift from the pan and cut into squares. For neat slices, warm your knife slightly and wipe clean between cuts.