Preheat the oven to 325°F(165C°). Line the bottom of a 9 inch (23cm) springform cake pan with parchment paper and set aside.
Put the sugar and lemon zest in a big bowl and mix with a fork or rub with your hand. Add in the flour and mix well.
Add the cubed butter and rub them with your fingers until they look like crumbs. Then add in the lemon juice and start to stir with a spatula.
Stir in the white chocolate and mix it evenly. The dough is stiff. Place the dough in the cake pan and spread it slowly. (You can use your measuring cup and press to the spread dough for an even thickness)
Bake for 22-25 minutes or until the edges become darker, and the top is lightly golden brown but still soft. Take the cookie cake from the oven and keep it for 10 minutes in the cake pan. By using small spatula or knife, make sure the cookie cake is not attached to the sides of the pan and gently open it. Don't remove the cake pan's bottom until the cookie cake is firm enough. While still warm, slice into 12 pieces.
Melt 30g (1 oz) white chocolate in the oven at 210°F (100°C). Place the chopped chocolate into an ovenproof dish. When the chocolate starts melting, stir the chocolate. Continue melting-stirring as needed. When the chocolate melts, dip a fork into the melted chocolate and drizzle it over the cookie cake.