Preheat oven to 160C and butter a bundt tin. Beat butter and sugar in an electric mixer until creamy. Add eggs one at a time, then add vanilla seeds and lemon rind. Meanwhile, sieve flours, xanthan gum and bicarbonate of soda into a bowl with a pinch of salt. With mixer on low speed, add half the flour mixture, then half the buttermilk.
Repeat, then spoon mixture into cake tin. Smooth top, then bake until cake is golden and an inserted skewer withdraws clean (40 minutes, if using large bundt tin; 45-50 if smaller tin). Cool in tin for 10 minutes, then invert onto a rack and cool completely.
Prepare the chocolate honey ganache; place cream and honey in a small saucepan on medium heat. Bring just to the boil and then take off the heat and add chocolate to hot mixture. Leave for a minute to allow chocolate to melt, then stir until mixture is smooth.
Set aside to cool completely and chill in the fridge until cold and thick. Pour over the top of cake and decorate with extra fresh blueberries. Serve immediately. Can be stored in an airtight container overnight, best eaten on the day.