Grease the pot of your slow cooker and line it with baking paper.
In a bowl, stir together the flour and baking powder.
In another bowl, cream together the butter and two sugars until light and fluffy.
Add the egg and vanilla extract and mix.
Stir in the dry ingredients.
It might look a bit dry but get your hands in and work it together so that it comes together like dough. When it is the right consistency the sides of the bowl will be clean.
Add the white chocolate chunks and mix through.
Add the halved raspberries and carefully mix through, avoiding crushing them.
Place the mixture in the slow cooker bowl, on the baking paper. Press it in so it is flat and even and reaches the edges.
Cook on high for 2-3 hours with a few pieces of kitchen roll under the lid to stop water dropping onto the dough.
The sides will start to come away from the pot when it is ready, and the top will still look a bit moist, but it will harden on cooling.
Take out the slow cooker pot and leave for a few minutes to cool.
Remove the cookie by pulling the sides of the baking paper. Place it on a cooling rack to cool. Or if you are going to eat it hot, add ice cream and sauce and dig in!