• In a large bowl, using an electric/stand mixer (or hand whisk, just double/triple the times) combine the butter and both sugars on medium speed for several minutes, until fully combined. Add the egg and vanilla and mix again.
• Add the flour, baking powder, white chocolate and fold until combined. The dough should be fairly solid at this stage (it ought to almost form a ball, albeit a very soft one!). 
• Cover and chill in the fridge at least 30 minutes.
• Preheat the oven to 200C/180C Fan/Gas 6. Line 2-3 large baking trays. Roll into about 12 balls (use a tablespoon to scoop balls of dough). Place on lined baking sheets. Bake 10-12 minutes, until just golden at the edges.
• Once removed, use a mug or bowl to press in the centre of the cookies - essentially creating a 'crust' around the edge and a dimple in the middle that the jam can go in without spilling!

Make the Jam

• Add the blackberries, lemon juice and sugar in a small saucepan over a medium/high heat. Stir until bubbling (only a few minutes), then add the cornstarch and stir - it should thicken a little. Cook a few more minutes. Remove from the heat and set aside to cool.
• Spoon this jam into the dimples in the cookies once relatively cool (doesn't need to be fully cold) and add any decorations (I add little white chocolate chips).