In a medium saucepan, combine 1/2 cup salted butter, 1/3 cup unsweetened cocoa powder, 2/3 cup milk chocolate chips, 2 cups granulated sugar, and 1 (12 ounce can) evaporated milk. Stir the mixture together over medium-low heat until the butter is melted and the ingredients are well incorporated.
Increase the heat to medium-high and bring the mixture to a gentle boil, stirring constantly. Once boiling, immediately reduce the heat to low and continue simmering for 7 minutes. Keep stirring to prevent the sauce from sticking or burning.
After 7 minutes, remove the saucepan from the heat and stir in 1 teaspoon vanilla extract.
Carefully pour the mixture into a blender. Be sure to vent your blender according to the device instructions for blending hot items. Blend on high for 2 minutes, until smooth and glossy.
Serve the hot fudge sauce immediately over ice cream, cakes, or other desserts. Alternatively, transfer the sauce to a heat-safe, airtight container and refrigerate for up to 2 weeks. Reheat gently before serving.