How to Make It
Line a baking sheet with a 10-by-15-inch sheet of parchment paper. Toast the fennel seeds in a small skillet over high heat, stirring occasionally, until lightly browned, about 3 minutes.
In a small saucepan, cook the sugar and water over moderate heat until richly browned, about 13 minutes. Remove from the heat and stir in the fennel. Immediately pour the mixture onto the baking sheet and spread it 1/8 inch thick with a heatproof spatula. Break the warm brittle into rough 1-inch pieces.