Line an 8x8-inch (20x20 cm) or 9x9-inch (23x23 cm) square baking pan with parchment paper, leaving an overhang on two sides to lift the fudge out later. Lightly grease the parchment with butter or nonstick spray.

Place the mint Oreos in a zip-top bag and crush into chunky pieces using a rolling pin or the bottom of a sturdy mug. Repeat with the candy canes, creating a mix of fine crumbs and slightly larger pieces. Set aside.

In a medium saucepan over low heat, combine the white chocolate chips, sweetened condensed milk, and butter. Stir constantly with a heat-safe spatula, scraping the bottom and sides, until the chocolate is fully melted and the mixture is smooth and glossy. If it starts to overheat or melt unevenly, remove from the heat for 30–60 seconds and keep stirring.

Remove the pan from the heat. Stir in the vanilla extract, a pinch of fine sea salt, and 1 teaspoon of peppermint extract. Taste a tiny amount carefully (it will be hot) and add up to an additional 1/2 teaspoon peppermint extract if you prefer a stronger mint flavor.

Let the fudge mixture sit off the heat for 1–2 minutes to cool slightly. Gently fold in about two-thirds of the chopped Mint Oreos, stirring just until evenly distributed so the cream filling doesn’t fully streak through the fudge.

Pour 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 thick, gently nudge it into place rather than pressing hard.

While the fudge is still warm and soft, sprinkle the remaining chopped Oreos evenly over the top and press them in gently. Scatter the crushed candy canes and festive sprinkles over the surface, pressing lightly so they adhere.

If using, melt the semi-sweet or dark chocolate chips in a microwave-safe bowl in 15–20 second bursts, stirring between each, until smooth. Drizzle over the fudge in zigzags, or use a toothpick to gently swirl it into the top layer for a marbled effect.

Cover the pan loosely with foil or plastic wrap and refrigerate for at least 2–3 hours, or until the fudge is fully set. For the best texture and cleanest cuts, chill for 4 hours or overnight.

Use the parchment overhang to lift the fudge out of the pan onto a cutting board. Run a sharp knife under hot water, wipe dry, and cut into 1–1 1/2 inch squares, wiping the knife between cuts. Serve slightly chilled or at cool room temperature.