1. Place a 4-cup mold in a freezer until well chilled.

2. Make the raspberry jelly: Place raspberries and sugar in a heatproof bowl. Cover bowl with plastic wrap and place over (but not touching) simmering water. Let heat release juices from berries for about 30 minutes.

3. Place a fine mesh sieve over a large liquid measuring cup. Strain berry mixture into measuring cup. Add lemon juice and enough water to berry mixture to make 2 cups.

4. Place gelatin in heatproof bowl. Add enough berry mixture to just cover gelatin; let stand 10 minutes. Place bowl over (but not touching) simmering water; stirring occasionally, until gelatin has melted. Stir in remaining berry mixture and strain through a fine mesh sieve into a liquid measuring cup or pitcher; set aside.

5. Make the blancmange jelly: Place gelatin in a heatproof bowl and add 2 tablespoons water, sugar, and lemon zest; let stand for 10 minutes. Place bowl over (but not touching) simmering water and heat until gelatin has melted.

6. Remove bowl from heat and stir in 1/4 cup plus 2 tablespoons water and milk. Strain through a fine mesh sieve into a liquid measuring cup or pitcher.

7. Remove mold from freezer. Pour a 1/2-inch layer of blancmange jelly into mold and transfer to refrigerator to set, about 15 minutes. Repeat process, alternating with raspberry jelly. When all layers are complete, transfer to refrigerator and let chill for 2 hours.

8. Moisten a serving plate with water. To unmold, set the mold in a bowl filled with warm water until the jelly around the edge has melted just slightly (this can take anywhere from 2 to 30 seconds). Invert mold onto prepared serving plate. Serve immediately with whipped cream and raspberries, if desired, or refrigerate until ready to serve.