Preheat oven to 175°C/325°F/Gas mark 3. Prepare baking tin with grease proof paper.
Mix together flour, baking powder, xanthan gum and salt and set aside.
Gently melt chocolate with water and sugar together in non stick pan over a low/medium heat until chocolate has melted.
Beat eggs and oil together and add vanilla then add melted chocolate mixture. Mix until fully combined.
Combine wet mixture to flour mixture folding in gently until fully combined ensuring that there are no lumps.
In a separate bowl blot Raspberries with a paper towel to remove any excess water and coat with 2 tablespoon of plain flour. Then add raspberries to wet mixture folding in evenly.
Add to prepared baking tin and spread evenly. Randomly spoon dollops of the jam on top of mixture and with the back of the spoon spread over the top gently making a swirl effect.
Place in the oven for approximately 25-30 minutes. Checking with a skewer when it almost comes out almost clean it is ready.
Move to cooling rack leaving Blondie in the tin to cool. Drizzle melted white chocolate to decorate if desired. when almost cool place in the fridge for 30 minutes.
Once cooled cut evenly into your desired sized squares.