Line a 9x5 loaf pan with a large piece of foil that has been folded in half, and then generously grease the foil with butter.

In a heavy saucepan, melt butter over medium-high heat. Add brown sugar and evaporated milk, and bring to a rolling boil. Allow to boil for 1 minute.

Remove from heat, and stir in salt, powdered sugar, and vanilla. Fold in pecans, if desired, and then spread in prepared loaf pan. Cool uncovered in refrigerator for about 30 minutes.

Once it has set, lift the foil out of the pan, and cut into squares. Store in an airtight container in the fridge.