To make the lemon cake layers:

1. In a mixer with a beater attachment, mix butter with sugar on medium speed until pale and creamy, about three minutes. Scrape down to the bottom of the bowl once during mixing to make sure no butter or sugar is stuck at the bottom.
2. Meanwhile, zest the lemons using a zester or the fine side of a grater. Add to the butter and sugar mixture and mix to combine. Add the eggs one at a time, mixing on the lowest speed after each egg until incorporated, about 30 seconds.
3. Add vanilla and mix on lowest speed for about 30 seconds to combine.
4. In a bowl sift and whisk together flour and baking powder. Add half of the mixture to the mixing bowl and mix on the lowest speed to incorporate.
5. Add milk and sour cream and mix into the batter. Add the rest of the flour mixture and mix only until just combined. The batter will be runny and pale in colour.
6. Grease three 6" pans and divide the batter between them. Tap the pans to flatten the batter or use a spatula to spread it flat.
7. Bake at 160ºC or 325ºF for 30 minutes or until a toothpick inserted into the middle of the cake comes out clean.
8. Let cakes cool in their pans for 5 minutes. Use a spatula to loosen cake from the edges of the cake pan and then turn onto a wire cooling rack. Leave to cool completely.

To make the lemon curd:

1. Put all of the lemon curd ingredients in a small saucepan over low or medium-low heat (depending on your stovetop). Bring to a simmer, whisking constantly until it thickens, which will take about 30 minutes. To test it, dip a spoon into it and it should coat the back of the spoon. Run your finger through that coating and you should leave a clear trail behind in the curd.
2. Pour the lemon curd through a sieve into a bowl, pushing through the sieve with a spoon if necessary.
3. Cover the bowl with plastic wrap, pushing it down to sit on the surface of the curd. Refrigerate for at least two hours and the curd will thicken more.

To make the blueberry buttercream:

1. Put the blueberries, water and white sugar in a small saucepan. Bring to a simmer over medium heat and mash the blueberries with a potato masher or fork. Simmer until the liquid thickens, about 5 minutes.
2. Pour the cooked blueberries into a sieve placed over a bowl and push the berries against the sieve using a spoon or spatula. Discard the blueberry skins and leave the blueberry puree in the bowl to cool.
3. Using an electric mixer, mix butter and powdered sugar on the lowest speed until smooth, about 4 minutes. Add vanilla and mix to combine.
4. Add the cooled blueberry puree a teaspoon at a time, mixing until the buttercream is easy to stir but still holds its shape.
5. Fit a piping bag with an open star piping tip like an 8B. Fill the bag no more than 3/4 full with blueberry buttercream.

To put it all together:

1. Pipe a dot of buttercream onto the middle of a plate or cake board and press your first layer of cake onto it. Pipe rosettes of the blueberry buttercream around the outer edge of the cake and spoon lemon curd into the middle.
2. Flip your next cake layer upside down and lower it onto the piping. Repeat the lemon and blueberry filling on this layer and then place the final layer on top, also upside down. Repeat the lemon and blueberry filling and gently press a fresh blueberry into the middle of each rosette.