Prepare an 8-inch square pan by lining it with foil and lightly greasing the surface.
In a medium saucepan over low heat, combine the chocolate chips, sweetened condensed milk, cocoa powder, and butter. Stir gently for about 5 minutes, until the mixture is glossy and bubbling softly around the edges.
Remove from heat and whisk in the vanilla extract and salt until fully incorporated.
1 tsp pure vanilla extract, 0.25 tsp kosher salt
Pour the warm fudge into the prepared pan, smooth the top with an offset spatula, then chill in the refrigerator for at least 2 hours. Lift the set fudge out using the foil and cut into 36 squares.
Notes
Lining the pan with foil and spraying it lightly ensures clean release.Chill the fudge fully before slicing to get neat edges. Store leftovers in an airtight container in the refrigerator for up to 2 weeks; freeze for up to 3 months.