Preheat oven to 375ºF/190ºC. Spray 12 muffin cups with nonstick cooking spray or line with cupcake liners. Set aside.
In a large bowl, mix together flour, sugar, baking powder, and salt. Set aside.
In a medium bowl, whisk together egg, milk, oil, vanilla extract, lemon juice and zest. Pour the egg mixture into the flour mixture and fold with a rubber spatula or wooden spoon just until combined. Stir in 115 grams chopped white chocolate.
Divide mixture evenly between muffin cups. Bake for 15-20 minutes or until a toothpick inserted into the center of a muffin comes out clean. Remove from oven. Allow to cool 10 minutes in pan, then gently transfer to a wire rack to cool completely. Once cooled, drizzle melted white chocolate over the muffins using a spoon or a piping bag.
Muffins are best the same day they are made, but can be kept in an airtight container, at room temperature or in the fridge, for up to 3 days.