Heat the oven to 180C, 160 fan, gas 4. Line the base of a 20cm deep, round cake tin with non-stick baking paper.
First make the crumble topping. Put all the topping ingredients in a small bowl and add 25g of the butter and 25g of the brown sugar. Rub together until crumbly. Spread evenly onto a lined baking sheet and bake for 10-15 minutes until golden brown and crisp. Cool.
Reduce the oven temperature to 140C, 120C fan, gas 1.
Put the remaining butter, brown sugar and the golden syrup in a medium pan and heat gently, stirring, until melted and smooth.
Beat the eggs and milk into the sugar mixture in the pan. Measure the flour into a medium bowl and lowly whisk in the contents of the pan until you have a smooth batter.
Pour the batter into the cake tin and bake for about 1 hour 45 minutes or until a skewer inserted into the centre comes out clean. Cover the top of the cake loosely with foil after 1 hour.
Leave in the tin for 10 minutes then turn out onto a wire rack to cool completely.
Meanwhile make the filling. Beat together the mascarpone cheese with the custard or the cream and vanilla, and chill.
Tip the rhubarb compote into a wide sieve over a bowl and leave to drain for 10 minutes. Stir 1 tbsp of the cordial into the drained rhubarb.
Put the remaining cordial in a small saucepan with the rhubarb liquid and bring to the boil. Bubble gently for about 5-7 minutes until it is thickened and syrupy. Cool.
Cut the cooled cake in half. Put the base on a serving plate and drizzle generously with some of the rhubarb and elderflower syrup.
Spoon on the whipped mascarpone cream and top with the rhubarb.
Pierce the top of the cake with a fine skewer and spoon over more of the syrup. Lift the top onto the cake and press on the crumble topping. Dust lightly with icing sugar and serve.
The cake will keep in the fridge for 2-3 days. Allow 15-20 minutes at room temperature before serving.
You can make half the quantity of the crumble for this cake, which is literally about a teaspoon of each ingredient. You can use any flour you have, any seeds and any nuts. Whatever is on hand.
If the ingredients are halved the cake can bake into a deep 18cm sandwich tin or a 500g loaf tin. Bake for about 45mins to 1 hour or until a skewer comes out clean. Cool and split in half.
The filling can be mascarpone or any soft cheese, even a soft, fresh goats cheese. Make up half the quantity.
Bonne Maman Apricot Compote can also be used as an alternative to Rhubarb or simply use a Bonne Maman Conserve such as Fig, Apricot or Plum.