For the shortbread, beat the butter with the granulated and icing sugars until light and fluffy, about 3 minutes. It’s an important step to beat the butter and sugar well together when making shortbread – this ensures the shortbread will hold together and when you take a bite it will ‘snap’ and then melt away.
In a separate bowl, sift the flour, cocoa powder and salt. Add this to the butter mixture and stir until evenly blended. Shape the dough into a disc, wrap in plastic and chill until firm, about an hour.
Preheat the oven to 325 F and line a baking tray with parchment paper.
Unwrap the dough and knead a little just to soften, making it easier to roll. Roll the dough out on a lightly floured work surface until it is just over 1/8-inch thick. Cut out cookies using a 2 ½ -inch fluted cutter and place carefully on the baking tray, re-rolling the dough if needed.
Bake the cookies for 13-15 minutes until they lift easily when lifted with a spatula. Cool the cookies on the tray before removing.
To assemble the desserts, whip the cream until it holds a soft peak, then whisk in the sugar, skim milk powder (optional, if you wish to whip the cream ahead of time – it will keep the cream stable for a full day) and vanilla. Spoon the cream into a piping bag fitted with a large star tip.
To build the desserts, place a cookie on a plate and pipe a bit of cream in the centre. Arrange 3 to 4 raspberries around the cream (the cream will hold them in place) and top with a second cookie. Pipe cream onto the centre of this cookie and arrange 3 to 4 more raspberries around. Top with a final cookie and dust with icing sugar. Repeat with the remaining cookies.
The desserts can be assembled up to 4 hours in advance of serving and stored chilled. The cookies themselves will keep up to a week in an airtight container.