How to Make It
Set a 12-by-18-inch sheet of parchment on a large baking sheet and oil the paper. Have a kitchen towel ready.
In a medium saucepan, cook the brown sugar with the lemon juice over moderately high heat, stirring occasionally, until the sugar melts, about 2 minutes. Reduce the heat to low and cook, stirring constantly, until richly browned, 3 to 4 minutes. Working quickly, stir in the puffed rice until coated, then immediately scrape the mixture onto 1 end of the baking sheet.
Fold the paper over the rice, cover with the kitchen towel and roll into a tight 3-inch log. Unwrap and let cool to room temperature, about 15 minutes. Using a serrated knife, gently slice the log crosswise 1/2 inch thick and serve.