1.Preheat oven to 160°C/325°F. Grease a 20cm x 30cm rectangular slice pan; line base and long sides with baking paper, extending the paper 5cm over sides.
2.Beat butter, extract, sugar, eggs, sifted flours, milk and ground hazelnuts in a medium bowl with an electric mixer on low speed until just combined. Increase speed to medium; beat for 2 minutes or until mixture is smooth and changed to a paler colour.
3.Spread mixture into pan. Top with plums, cherries and nuts.
4.Bake for 1 hour or until a skewer inserted into the cake comes out clean. Cool cake in pan.
Serve drizzled with custard. Cake can be made 2 days ahead, store in an airtight container.