On a silicone baking mat, set up a frame of caramel rulers 6½ by 12 in/17 by 30 cm.
Combine the sugar, evaporated milk, raspberry purée, and lemon zest in a heavy-bottomed saucepan and bring to a boil, stirring constantly. Add the glucose syrup while continuing to stir. Cook, stirring, to 230°F/110°C.
Add the butter and continue cooking to 245°F/118°C