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Heat oven to 160C/140C fan/gas 3. Grease and line a 20cm square tin with baking parchment. Put the butter, sugar, eggs, yogurt, flour and baking powder in a large bowl and beat with an electric whisk until smooth. Tip into your cake tin and smooth the top. Bake for 45-50 mins until risen and golden, and a skewer inserted comes out clean. Leave to cool for 10 mins in the tin, then turn out onto a wire rack to cool completely.
Grind the lavender to a powder using a pestle and mortar. To make the butter icing, beat the butter and icing sugar together until pale and fluffy. Divide the mixture into 2 bowls, add the lemon zest to one batch and the lavender to the other, then mix each to combine. Transfer the icings to 2 piping bags.
When the sponge has completely cooled, use a large serrated knife to remove the top and give you a flat surface. Flip the cake over so the flattest side becomes the top. Carefully trim off the edges, then cut the sponge into 16 squares. Use a ruler if you want them to be the same size. Brush the top and sides of each sponge square with a little jam. Pipe a small dome of butter icing onto each sponge, 8 with lavender icing and 8 with lemon. Chill the cakes for at least 1 hr.
Sieve the fondant icing sugar into a large bowl, and add enough water to make a thick but pourable icing. Spoon a little white icing into a sandwich bag or piping bag to use later. Divide the remaining icing into 2 bowls, dye one batch lemon yellow and the other pale lilac.
Remove the sponges from the fridge and transfer to a wire rack, sitting the rack on a tray to catch any drips. Spoon a little icing over each sponge, lilac for the lavender-flavoured fancies and yellow for the lemon. Use a small palette knife to spread the icing down the sides of the sponge to completely cover each one. Leave to set for 30 mins, then cover the cakes with a second layer of icing to give a smooth surface. Drizzle the white icing over the cakes and decorate with a little lemon zest and lavender buds. Leave to set for 1 hr at room temperature before serving. Will last 2-3 days in a tin.