1.Position oven shelves; preheat oven to moderate. Grease deep 22cm-round cake pan; line base and side with baking paper.
2.For custard, combine custard powder and sugar in small saucepan; using wooden spoon, gradually stir in milk. Stir over heat until mixture boils and thickens. Remove from heat, add vanilla and butter; stir until butter melts. Cover surface of custard completely with plastic wrap to prevent skin forming; cool to room temperature while you continue with the rest of the cake (do not refrigerate as mixture will not be spreadable).
3.Beat butter and sugar in medium bowl with electric mixer until light and fluffy; add eggs, one at a time, beating well between additions. Using wooden spoon, stir in flour, almond meal, custard powder and milk.
4.Using metal spatula, spread half of the cake mixture into prepared pan, top with half of the rhubarb and pear.
5.Spread custard over fruit; spread remaining cake mixture over custard, top with remaining rhubarb and pear.
6.Bake cake in moderate oven about 1 hour. Stand cake 5 minutes then turn onto wire rack; turn top-side up, brush top with warm jam. Cool.
Large firm strawberries can be sliced lengthways and substituted for the rhubarb; apple or nashi can be substituted for the pear; or a combination of any of these fruits (including the rhubarb) can be used. Any jam can be substituted for apricot jam. Warm jam in the microwave oven for about 30 seconds on HIGH (100%); strain while warm. Any ground nuts can be substituted for almond meal; use a blender or processor to grind nuts finely. Cake will keep for 1 day in an airtight container in the refrigerator.