Preheat the oven to 155°C (310°F) / 135°C Fan / Gas Mark 3. Lightly oil two 20cm springform cake tins and line the bases with baking parchment.
In a large mixing bowl or a stand mixer with the beater attachment, whisk the eggs, oil, and muscovado sugar until well combined.
Add the plain flour, spelt flour, baking powder, and salt to the bowl.
Mix on a low speed until the are just combined.
Do not overmix.
Pat 6 pineapple rings dry with kitchen paper.
Arrange them in a single layer in the base of one of the prepared tins.
Place a glacé cherry in the center hole of each pineapple ring.
Divide the cake batter evenly between the two tins.
Bake for 35–40 minutes, or until a skewer inserted into the center comes out clean.
Let the cakes cool in their tins for 10 minutes before turning them out onto a wire rack.
While the cakes are baking, prepare the glaze. Dissolve the granulated sugar in 2 tbsp (30 ml) of boiling water, then stir in the dark rum.
As soon as the cakes are on the wire rack, prick them all over with a skewer and brush generously with the rum glaze.
To make the filling, place 80g (2.82 oz) of the butter in a small, clean saucepan with the vanilla seeds and the scraped pod. Melt over a low heat.
Continue to heat as the butter sizzles and froths.
Once the sizzling subsides and the butter turns a golden brown with a nutty aroma, immediately remove it from the heat. Allow it to cool completely, then discard the vanilla pod.
In a large bowl or stand mixer, beat the remaining 170g (6 oz) of softened butter until creamy. Gradually beat in the cooled brown butter.
Add the sifted icing sugar and beat again until light, fluffy, and thoroughly combined.
To assemble, carefully trim the top of the plain sponge (the one without pineapple) to create a flat, even surface.
Place it on your serving plate and spread the entire brown butter filling over it. Gently place the pineapple-topped sponge on top. Decorate with edible flowers, if using, before serving.
