Arrange cinnamon graham crackers in a single layer with sides touching in bottom of a greased 10 x 15-inch jellyroll pan. Sprinkle walnuts evenly over crackers. In a heavy small saucepan, combine butter, brown sugar and cinnamon. Stirring constantly, cook over medium heat until sugar dissolves and mixture begins to boil. Continue to boil syrup 3 minutes longer without stirring. Pour over crackers. Bake at 400° for 8 to 10 minutes, or until bubbly and slightly darker around the edges. Cool completely in pan. Break into pieces. Store in an airtight container. Makes about 1 1/2 pounds.