Preheat the oven to 180°C, fan 160°C, gas mark 4. Grease a 20cm spring form cake tin and line with baking paper.
Place the butter, eggs, condensed milk, almonds, flour, baking powder and seeds in a large bowl. Add the finely grated zest and juice of 1 lemon and beat together using an electric hand-held whisk until just smooth and pale.
Pour into the prepared tin and bake for 55-65 minutes, or until golden and a skewer comes out clean when inserted into the middle of the cake.
Make a lemon syrup by warming together the juice of 3 lemons with 100g of the icing sugar in a small pan over a gentle heat.
When the cake comes out of the oven, make holes all over it with a skewer, then gradually spoon over the syrup, allowing it to sink into the cake. Leave to cool in the tin.
To make the icing, gradually add the juice of the remaining lemon (about 2 tablespoons - you may not need all of it) to the remaining 100g of icing sugar, stirring until you have a thick pouring consistency.
When the cake is cool, transfer to a serving plate and drizzle over the lemon icing.
Cook's tip: If the cake starts to brown while cooking, cover the top with a layer of baking parchment for the remaining cooking time.