Delicious served still warm as a cake on its own, or with yoghurt or whipped cream as a dessert. A dollop of lemon curd would be great too. It is still very good 2-3 days later as well.
Heat the oven to 180C. Line a 23cm square cake tin with baking paper. Lightly toast the flaked almonds for the topping.
Place the eggs, sugar, butter and vanilla extract in the bowl of a food processor and process for 1 minute. Scrape down the sides of the bowl.
Sift the flour and baking powder together and add to the food processor with the lemon zest. Process for 10 seconds.
Add the water and ground almonds and process for a further minute. Pour mixture into the prepared tin and place in the oven. Bake for 40 minutes.
For the topping, melt the butter in a heavy-based saucepan over low heat. Add the sugar, vanilla extract, salt and milk. Bring slowly up to the boil and cook for about 1 minute until the mixture thickens. Remove from the heat and stir in the almonds.
Remove cake from the oven and spread almond topping over it. Return to the oven for a further 10 minutes or until a skewer inserted in the centre of the cake comes out clean.
Leave to cool in the tin for 15 minutes, but after 5 minutes run a knife around the tin to prevent the topping from sticking. Remove to a wire rack to cool completely. Store in an airtight container for 2-3 days.