Preheat oven: 180°C / 160°C (fan) / Gas Mark 4 / 350°F
In a large heatproof bowl, melt the butter for 30-50secs in the microwave and beat in the sugar until smooth and thickened.
Thoroughly beat in the egg and vanilla extract until well combined.
Sieve in the flour and cornflour, stirring until just combined.
If using, stir in the white chocolate chips.
Pour the blondie mix into a greased and lined 8-inch square pan **, smoothing the top gently.
Using a teaspoon, dollop on the jam at spaced intervals and gently swirl through.
Press 16 mini jammie dodgers into the top of the blondie mix, spacing evenly in 4 rows of 4 biscuits.
Bake for 25-30mins until the top is lightly golden and there is only a slight wobble in the middle. Allow to cool in the tin before removing and cutting into squares.
* The cornflour used in this recipe is a white fine flour, not a yellow coarse flour. However, sometimes this is called cornstarch and the yellow version is called cornflour, e.g. in the US, so be sure to pick the white one!** You can swap the 8-inch square pan for a 9-inch square pan and reduce the baking time by 5mins.