Sweet Potato Cake With Oak Panna Cotta & Buttermilk Sorbet

Using vegetables – like this one with sweet potato and oak panna cotta – in desserts is not everyone’s cup of tea. I like it, as it’s not too sweet and has a lovely savouriness to it.

Serves 8

Fundamentals:

  • 24 sweet potato crisps (3 per serving)
  • 12g pistachio powder (1.5g per serving)
  • 53g buttermilk sorbet (1 x 6.5g quenelle per serving, shaped using a hot tablespoon)

Sweet potato cake

  • 90g unsalted butter, to make beurre noisette
  • 90ml sunflower oil
  • 2 whole eggs
  • 200g brown sugar
  • 200g self-raising flour
  • 5g baking powder
  • 300g sweet potato, finely grated
  • 1/2 banana, mashed
  • 1/2 orange, the zest

Oak Panna Cotta

  • 3 leaves gelatin
  • Ice bath, to hydrate
  • 210 ml full-cream milk
  • 219 ml fersh honey
  • 5g lemon thyme
  • 10g smoking wood chips
  • 45ml brandy

1. Preheat the oven to 175C. Line a baking tray with silicon paper. To prepare the beurre noisette for this recipe, place 90g unsalted butter in a pot over medium heat. Continue to whisk the butter as it melts and begins to foam. Keep the butter on the heat until it begins to brown and the impurities begin to rise to the top. At this stage, strain the butter through muslin cloth and set the liquid aside to cool. Once cooled, measure out 50g of the beurre noisette and combine with the oil.

2. In a heatproof bowl over simmering water, whisk the eggs and brown sugar until pale in colour and aerated. Whisk in the butter and oil mixture slowly. Sift 180g of the flour with the baking powder, and fold into the butter and sugar mixture, and then mix in the remaining 20g flour with the finely grated sweet potato. Fold the mashed banana into the cake mixture. Fold in the sweet potato, and, lastly, the orange zest. Bake in the preheated oven for 30-40 minutes. Once cooled, slice 8 squares, 4 x 4cm, for serving.

3. Hydrate the gelatin leaves in the ice bath. Heat the milk, cream, honey, lemon thyme and wood chips in a pot, but do not boil. Once hot, remoce from the heat and leave to infuse for 30 minutes, then heat the mixture again, and stir in the soaked gelatin leaves and the brandy. Pour through a chinois (fine-meshed conical sieve) to strain the mixture into a bowl, and cool over the ice bath (add extra ice to the ice bath use to hydrate the gelatin).

4. Once cooled, pour into 4cm-diameter round moulds and freeze. When frozen, remove from the mould and store in the fridge.

5. To assemble, place a square piece of cake on each plate with the panna cotta. Spoon a little pistacio powder onto the plate and position the sorbet on it so that it won’t slide around. Separate the components by placing the sweet potato crisps between them, and finish with more pistacio powder.

How to make the pistacio powder 

  • 12g pistacio nuts

Roast the pistacio nuts in a pan until they are slightly toasted and begin to release their natural oils. Remove from the pan and transfer to a blender. Blend into a fine dust and reserve in an airtight container.

How to make the sweet potato crisps 

  • 1 large orange-fleshed sweet potato
  • olive oil, for brushing

Rinse the sweet potato. Top and tail, and place on the gravity slicer, slicing on setting 1 or until thin enough to be almost transparent. Brush with olive oil and set on dehydrator racks between Silpat mats to weigh them down. Dehydrate at 54C for 48 hours or until properly crispy.

How to make the buttermilk sorbet

  • 110ml milk
  • 110ml buttermilk
  • 1/2 vanilla pod
  • 130g castor sugar
  • 40ml water
  • 70ml glucose
  • 110ml plain yoghurt
  • 360ml buttermilk
  • 1 lemon, the zest
  • 1 lime, the zest

Heat the milk, buttermilk and vanilla pod together. In a separate pot, heat the castor sugar, water and glucose, making sure that the sugar dissolves. Add the sugar mixture to the vanilla pod mixture. Lastly, add the yoghurt and buttermilk with the lemon and lime zest. Churn in an ice cream machine, and freeze until needed.

About this recipe

We extracted this recipe from the cookbook called Mile 8, which David Higgs, celebrity chef and co-owner of Marble restaurant, recently released.

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