Pineapple and rum upside-down cake with brown butter filling

Browning the butter first gives the filling a flavour which is rather intriguing, while adding the vanilla at this browning stage produces a more pronounced taste, too. We use some spelt flour in the cake batter as it gives a delicious background nuttiness which echoes the flavour of the brown butter. If you have only got one 20cm springform tin, just bake the cakes one after another in the same tin. I think that has been said before …

SERVES 10–12
For the cake
250g sunflower oil, plus more for the tins
5 eggs, lightly beaten
250g light muscovado sugar
100g plain flour
100g spelt flour
1 tsp baking powder
pinch of sea salt
234g can of pineapple rings
6 glacé cherries
For the rum glaze
1 tbsp granulated sugar
6 tbsp dark rum
For the filling
250g unsalted butter, softened
1 vanilla pod, split, seeds scraped out
200g icing sugar, sifted
To decorate
1 punnet of edible flowers (optional)
Preheat the oven to 155°C/fan 135°C/gas mark 3. Oil two 20cm springform cake tins and line the bases with baking parchment.
For the cakes, whisk the eggs, oil and sugar in a large mixing bowl (or use a food mixer fitted with the beater attachment), then add the plain flour, spelt flour, baking powder and salt and mix until just combined.
Pat 6 pineapple rings dry with kitchen paper, then make a layer of them in the base of one of the prepared tins. If the pineapple rings are very large, you may need to slice them in half. Sit a cherry in the hole of each.
Now divide the cake batter between the two tins and bake for 35–40 minutes or until firm to the touch; a skewer should come out clean. Allow to cool in the tins for 10 minutes, then carefully release from the tins and turn out on to a wire rack, pineapple-side up. (Or bake the cakes in the same tin, one after another, adding the base layer of pineapple and cherries to just one of the cakes.)
Meanwhile, to make the glaze, simply dissolve the sugar in 2 tbsp of boiling water before adding the rum, then prick the sponges with a skewer while they’re still warm and brush the glaze all over using a pastry brush.
For the filling, weigh 80g of the butter into a small, scrupulously clean saucepan with the vanilla seeds and pod and melt it over a low heat. The butter will start to sizzle and froth up; watch it closely. Once the sizzling has died down and the butter has turned a golden brown colour, a nutty aroma will be released. Immediately remove it from the heat. Allow to cool, then remove the vanilla pod.
Beat the remaining butter in a large mixing bowl (or use a stand mixer fitted with the beater attachment), then gradually beat in the cooled brown butter. Now add the icing sugar and beat again until thoroughly combined.
To assemble the cake, trim the top of the sponge without pineapples to create a flat surface, then spread on all the brown butter filling. Sit the pineapple sponge on top and scatter with edible flowers, if you like.