Preheat oven to 180C. Grease and line base and sides of a standard sized loaf tin.
Beat butter and caster sugar until pale and fluffy, the beat in eggs, one at a time; it may look a bit curdled, but don't worry. Gently fold in lemon zest and flour, alternating with the yoghurt. Using a spatula, spread half the mixture into the prepared loaf tin. Cover with grapes, and spread over the remaining batter. Bake for 45 minutes or until golden brown and a skewer inserted into the centre of the cake comes out clean.
Remove cake from oven, but leave it in the pan for 15 minutes, then transfer to a wire rack to cool completely.
Meanwhile, crystallise some of the grapes. Beat the egg white until it starts to bubble and froth. Dunk a small bunch of grapes in the egg white and shake lightly to remove excess egg white. Place on a paper towel to whilst you do as many other remaining bunches as you like. Take the grapes off the paper towel and lightly toss in the caster sugar, and place on a tray lined with baking paper for a few hours to dry.
For the frosting, stir together the yogurt with the icing sugar and vanilla extract. Use a palette or butter knife to spread the frosting on the loaf.
Decorate with a mixture of crystallised grapes, fresh grapes and mini figs (if using).