Measure the frozen raspberries while frozen and then allow to thaw. Purée the thawed raspberries in a food processor until smooth. Pour the raspberry purée into a fine mesh strainer over a bowl. Using a spatula, press and scrape the purée through the strainer until all that is left is the seeds. Discard the seeds. You should have 1 cup of raspberry purée.
In a very small microwave safe bowl, such as a custard cup or a ramekin, combine the unflavoured gelatin and the water and then stir to combine. Set the gelatin aside.
Transfer the 1 cup of raspberry purée to a saucepan. In a separate bowl whisk together the sugar, cornstarch and kosher salt until there are no more lumps of cornstarch. Add the dry ingredients to the raspberry purée and whisk until combined. Turn on the heat to medium low and cook, whisking constantly, until the purée is bubbling and thickened. Once thickened remove from the heat.
Microwave the gelatin until melted. Start with 5 to 8 seconds and watch carefully; the gelatin mixture will melt quickly. Scrape the melted gelatin into the purée and whisk together. Pour the purée into a bowl and cover by placing a piece of cling film directly onto the surface of the filling. Chill until completely set, at least 6 to 8 hours or overnight if possible.