Preheat the oven to 180C fan/200C/400F/gas mark 6 and line an 8 inch baking tin.
In a large bowl/bowl of a stand mixer, add all the cake ingredients and combine until smooth. If very stiff, add a little more milk. Pour into tin and bake 20-25 minutes, until golden and a skewer inserted comes out clean. Let cool.
Once cool, make the frosting. Add the butter to a large mixing bowl. Beat with a whisk on high for a minute to soften/combine everything.
Add half the icing sugar, whisk on high. Then add the lemon juice and remaining half icing sugar and whisk everything on high until fluffy. Add passion fruit/icing sugar as required to meet your desired consistency. Spread over the cooled cake, top with a little lemon zest slice and serve!