In a heavy saucepan mix together the sugar, chocolate, and milk. Place the pan over high heat and stir constantly until the choclate is melted and the sugar is dissolved. Bring to a full boil, and lower the heat so the candy continues to boil gently, not vigorously. Stir no more. Put a candy thermometer into the center of the mixture and cook until the temperature reaches exactly 232 F. Meanwhile, butter a large platter (turkey size) and a flat pan about 11 by 13 inches. When the fudge reaches 232 F., pour it into the readied platter - do not scrape the pan, but let it drip out. Dot with 2 tablespoons butter and let the mixture cool until the platter feels cool underneath. Add vanilla. Take a large slotted spoon and start to stir the liquidy mixture - it will take about 15 or 20 minutes. You will see a steady change from dark to light color, from glossy to dull, from liquid to solid. When the fudge begins to get dull, add the nuts and mix in thoroughly. Put fudge into the large buttered pan and press into shape with the flat of your palms. Cut into squares; store in airtight container if there's any left.