Preheat the oven to 180C/170C (fan)/Gas 4. Lightly butter 2 x 20cm/8in springform cake tins and line with baking parchment.
Using an electric hand-held whisk, cream the butter, sugar and passion fruit in a large mixing bowl. In a separate bowl, sift together the rice flour, xanthan gum and baking powder.
Add a spoonful of the flour mixture and one egg to the butter and sugar mixture, beating in thoroughly. Add the rest of the eggs, one by one, adding a spoonful of the flour mixture each time.
Fold in the rest of the flour. Divide the mixture equally between the two tins and bake for 40-45 minutes, or until the top is golden-brown and a wooden skewer inserted into the centre of the cake comes out clean.
While the cakes are in the oven, make the buttercream by creaming together the lactose/dairy free butter, icing sugar and lime zest in a bowl until light and fluffy. Add the lactose/dairy free cream and beat until thoroughly combined. Set aside.
Shortly before the cakes are ready, make the syrup. Combine the passion fruit juice with the lime juice, then add the sugar, stirring to dissolve. ( You may need to heat in a small pan or give it a blast in the microwave in a heatproof jug.) Strain the syrup but reserve the passion fruit seeds.
When the cakes are baked, remove from the oven and prick them all over with a wooden skewer. Pour over the strained syrup and leave to cool in the tins.
When the cakes are cool, turn out one of the layers onto the serving plate and spread or pipe over the lactose/dairy free buttercream. Place the other cake layer on top.
Scatter the passion fruit seeds in the middle of the cake, and cut into slices to serve.