Line an 8×8-inch pan with parchment paper, leaving edges overhanging for easy lifting. Lightly grease parchment with butter to prevent sticking and set aside.

In a medium saucepan, stir sweetened condensed milk, unsweetened cocoa powder, butter, and pinch of salt over medium-low heat until evenly mixed, about 2 minutes.

Continue cooking, stirring constantly with a wooden spoon for 8–10 minutes until a glossy, thick texture forms and pulls away from the pan’s sides.

Remove pan from heat, then stir in rolled oats, vanilla extract, and chopped nuts until fully coated and evenly distributed throughout the chocolate mixture.

Transfer mixture into lined pan, pressing firmly into an even layer. Refrigerate for at least 2 hours until firm and sliceable.

Use parchment edges to lift fudge block, then slice into 16 squares. Serve chilled or at room temperature.