Line a 23cm x 28cm rectangular cake tin with baking parchment.
Combine the flour, malt powder and butter cubes in a food processor and pulse until the mixture resembles fine breadcrumbs.
Add in the caster sugar and pulse again until combined.
Tip the mixture into the lined cake tin and spread it out evenly with the back of a spoon. Then press the shortbread down firmly with your knuckles so that it is tightly packed in the tin.
Bake the shortbread for 30 minutes or until very light golden brown. Set aside to cool.
For the caramel layer:
Put the butter and sugar in a wide, heavy-based pan with the syrup, malt extract, condensed milk and salt, and heat gently, stirring to melt the butter.
Bring to a simmer and continue to cook for about 10-15 minutes, until thick and fudgy.
Pour over the shortbread and smooth with a palette knife, then begin to press the Maltesers into the caramel in a layer until the caramel is covered - you should have some left over.
Leave to set.
For the chocolate layer:
Melt the chocolate in a heatproof bowl over a pan of simmering water or in a microwave, and spread evenly over the set caramel. Crush the leftover Maltesers and sprinkle over the top of the chocolate.
Leave until solid, then turn out and cut into squares to serve.