Heat the oven to 180°C/160°C fan/gas mark 4. Grease and lightly flour a 20cm bundt tin. In a large bowl, whisk together the flour, lemon zest, bicarbonate of soda and salt to combine.
Add the sugar and butter to a stand mixer and use the paddle attachment on a low/medium speed to beat the two ingredients until the mixture is lighter in colour and fluffy. This will take a good 4-5 mins, so don’t rush it. Add the eggs one by one, beating well after each addition. Should the batter start to curdle, add 1 tbsp of the flour mixture.
Combine the sour cream with the lemon juice and 1 tsp vanilla extract. Now add the flour and sour cream to the cake batter intermittently, starting and ending with the flour, being sure not to overmix it but also ensuring that there are no streaks of egg or pockets of flour. The best way to check this after mixing is to use a rubber spatula to get right down to the bottom of the batter and gently fold through anything that isn’t combined.
Now pour your batter into the prepared bundt tin, transfer to the oven and bake for 55 mins. Insert a skewer to check that it is cooked through – it should come out clean, or at the very most have a few damp crumbs. If the mixture is still wet, return to the oven for an additional 5-10 mins. Cover with foil if the cake starts to get too brown. Once done, transfer to a wire rack and leave in the tin until completely cool.
Once cool, it’s time to ice the cake. To make the icing, in a medium bowl, whisk together the icing sugar and elderflower cordial until it is thick, smooth and runny. If it seems a little dry, add a splash more cordial, and vice versa if it’s a little too loose.
Turn out the cake onto a wire rack with kitchen towel or baking paper underneath (this will prevent icing from getting all over your kitchen worktop). Pour the icing over the cake so that it dribbles down the sides and covers the top, then leave to set. When it’s ready to serve, carefully transfer to whatever you plan to present it on and decorate with flowers.