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Preheat the oven to 180C/160C Fan/Gas Grease a non-stick 26cm/10½in bundt tin with cake release spray and make sure all corners are coated using a pastry brush (the cake is very moist and you'll struggle to remove it from the intricately shaped tin without careful preparation). Alternatively, use a 20x30cm/8x12in traybake tin, greased and lined with baking paper.
For the syrup, combine the sugar, 100ml/3½fl oz water, lime juice and rhubarb in a small saucepan. Heat gently until the sugar is dissolved and the rhubarb starts to soften, about 2 minutes. Remove half of the rhubarb using a slotted spoon and set aside to cool. Continue to gently cook the remaining syrup until thickened (about 15 minutes), then set aside.
For the sponge, mix the pistachios, almonds, polenta, salt and baking powder together in a bowl.
In a separate large bowl, beat the butter and sugar until light and fluffy. Add a large spoonful of the nut and polenta mixture and mix until combined. Beat in 1 egg, then another spoonful of the nut and polenta mixture. Repeat until all the eggs and nut mixture have been added. Fold in the reserved lime zest, the cooled rhubarb and the raspberries.
Tip the mixture into the prepared bundt tin and smooth the surface. Try to make sure fruit is evenly distributed and not touching the bottom. Push the raw rhubarb into the mixture.
Place the tin on a baking tray and bake for 1 hour and 10-15 minutes (1 hour if using a traybake tin), or until a skewer inserted into the cake comes out with just a few crumbs attached. Check after 45 minutes, if the cake is browning too much, cover loosely with aluminium foil.
When the cake is cooked, set aside for a few minutes to cool, then place a cooling rack over the bottom and turn the cake out. Leave to cool completely.
For the custard, add the cream, milk and vanilla bean paste to a saucepan. Heat gently until it starts to simmer. Remove from heat and set aside to cool a little.
Meanwhile, beat the egg yolks and sugar in a separate large bowl until smooth and pale. Slowly add the cream in a thin stream whisking all the time . Transfer to a clean saucepan and gently heat, whisking all the time, until it thickens. Pass through a sieve into a clean bowl and set aside to cool. Cover the surface with cling film to prevent a skin forming and refrigerate until needed.
For the decoration, make the frosted raspberries first. Scatter the caster sugar onto a small plate. Whisk the egg white until frothy, then using a small brush paint the whole fresh raspberries with the egg white then roll gently in caster sugar. Leave to dry, then repeat with the mint leaves.
Add the pistachios to the cooled syrup and mix until all the pistachios are coated. Remove and set aside. Reserve the remaining syrup.
Using a skewer, poke holes evenly around the cake and then pour the reserved syrup all over the cake, keeping back a tablespoon of syrup to make a glaze later. Set aside for 5 minutes to allow the syrup to sink in then transfer to a serving plate.
Put the custard in a piping bag with a small plain nozzle attached. Using a larger skewer or a thin knife, make two holes in each of the grooves on the bundt cake or a few in the top of the traybake. Pipe in a steady stream of custard to fill the holes (it doesn’t matter if some runs out). Keep any leftover custard to serve.
Add the icing sugar to the reserved syrup to create a pale pink glaze. Drizzle over the cake and along the grooves.
Use the frosted raspberries to cover up the holes where the custard was piped and decorate the cake with the pistachios and frosted mint. Scatter with lime zest and serve with any remaining custard in a bowl alongside.