Make the raspberry sauce. Add the raspberries to a 1-quart stainless steel saucepan and cook them over medium heat for 5-8 minutes until they've completely softened and released their juices. Pass the cooked raspberries though a sieve placed over a bowl or jug to remove the seeds. You should get about ⅓ cup (80ml) of thick raspberry sauce.
Make the curd base. Combine egg, egg yolk and sugar in another small saucepan and whisk to blend evenly so it is smooth. Add the corn starch and whisk until smooth. Add the lemon juice and whisk it in evenly, then whisk in the thick raspberry sauce.
Add the cubed butter and then place the saucepan over medium-low heat stirring the whole time for 7-10 minutes until it thickens up like pudding. It should read 85 degrees C on a thermometer and will just start to bubble up.
Once thickened, remove from the heat and pour the finished raspberry curd into a bowl or heat-proof container and cover it with a sheet of plastic wrap/cling film. Allow it to cool completely to room temperature, then chill it in the fridge for at least 2 hours or until completely set.
