Line an 8×8-inch (20×20 cm) baking pan with parchment paper, leaving overhang on two sides as handles. Lightly spray the parchment with nonstick spray or rub with a bit of butter, especially in the corners, to make it easier to lift out and slice the fudge.
Roughly chop the dried cherries so you get small pieces in every bite. If the cherries are very dry or tough, soak them in warm water or warm cherry juice for 5–10 minutes, then drain and pat very dry to avoid adding extra moisture. If using nuts, lightly toast them in a dry skillet until fragrant, then cool and chop.
In a medium heavy-bottomed saucepan, combine the dark chocolate, sweetened condensed milk, and butter pieces. Set over low heat and stir constantly with a heatproof spatula or wooden spoon until the chocolate is fully melted and the mixture is smooth, thick, and glossy. Keep the heat low to prevent scorching or grainy texture.
Remove the saucepan from the heat. Stir in the vanilla extract and fine sea salt until well combined. Taste a small amount carefully while warm and add a pinch more salt if you prefer a slightly saltier fudge.
Add the chopped dried cherries and the chopped toasted nuts (if using) to the warm fudge base. Gently fold until they are evenly distributed throughout the mixture.
Scrape the thick fudge mixture into the prepared 8×8-inch pan. Use an offset spatula or the back of a spoon to press it evenly into the pan, smoothing the top and pushing it into the corners for a level surface.
If desired, lightly sprinkle flaky sea salt over the warm fudge. Use just a small pinch for a subtle crunch and flavor contrast.
Cover the pan loosely with plastic wrap or foil, avoiding direct contact with the surface if possible. Refrigerate for at least 2 hours, or 3–4 hours / overnight for extra clean cuts, until the fudge is firm enough to slice.
Lift the firm fudge out of the pan using the parchment overhang and place it on a cutting board. Warm a sharp knife under hot water, wipe dry, and cut the fudge into 1- to 1½-inch squares, reheating and wiping the knife as needed for clean edges.
Serve the Dark Chocolate Cherry Fudge slightly chilled or at cool room temperature. Store in an airtight container at room temperature for up to 2–3 days if your kitchen is cool, in the refrigerator for up to 2 weeks, or freeze (well wrapped) for up to 3 months. Place parchment between layers to prevent sticking.