Combine granulated sugar, brown sugar and rum in medium size heavy saucepan.
Bring to boiling over medium-high heat. Continue cooking until temperature registers
275 degrees F on a candy thermometer, 15 to 25 minutes (soft crack state - mixture
forms pliable strands when drizzled from a metal spoon into a bowl of cold water).
Stir in walnuts. Quickly pour out onto prepared baking sheet, spreading
out with metal spatula as you pour. Let cool completely.
Break into 2-inch pieces.
Store in airtight container in cool, dry place for up to 2 months.