In a stand mixer with paddle attachment, beat together the butter and sugar. Add the vanilla and beat well.
Sift together the flour and cornflour, then add the sprinkles and mix well.
Add the flour mixture to the butter mixture in 3 parts, gently mixing between each until just incorporated.
Flatten the dough into a disk in some plastic wrap and sit in the fridge to firm up for about 1/2 an hour.
Preheat the oven to 180C / 350F / 160C fan forced. Line a large baking tray with baking paper.
Roll 1/2 tablespoons of mixture into balls and then press down lightly with a floured fork on top of each and sprinkle with a few extra sprinkles.
Bake for 15 minutes until just starting to brown on the edges, then transfer to a wire rack to cool.
FOR THE RASPBERRY FILLING
Heat the raspberries in the microwave for 30 seconds. Push them through a strainer until you have about 6 teaspoons of raspberry juice. Discard the seeds.
Add the raspberry juice, butter, icing sugar and vanilla to a bowl and beat with a handheld mixer (or in a stand-mixer) until light and creamy. Scrape down the sides of the bowl as necessary. Add some pink food colouring to intensify the colour if using and mix well.
Spread or pipe some buttercream onto the flat side of half of the cooled cookies, then top with another cookie.
For best results you should always weigh ingredients like flour and sugar. Kitchen scales are relatively cheap but if you can’t weigh the ingredients, use the spoon and level method (don’t scoop).
Isn't that a lot of corn flour (cornstarch) - The corn flour plays a very important role in this recipe by keeping these cookies meltingly tender.