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.
3. Add the eggs one at a time, mixing on the lowest speed after each egg until incorporated, about 30 seconds.
4. Add vanilla and mix on lowest speed for about 30 seconds to combine.
5. In a bowl sift and whisk together flour, baking powder and salt. Add half of this mixture to the mixing bowl and mix on the lowest speed to incorporate.
6. 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.
7. Grease three 6" pans and optionally, line with parchment paper rounds to prevent sticking. Divide the batter between them. Set aside 8 raspberries and divide the rest between the cake pans. Nudge the raspberries under the surface of the batter with a spatula.
8. Bake at 160ºC or 325ºF for 30 minutes or until a toothpick inserted into the middle of the cake comes out clean.
9. 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 lemon buttercream:

1. Using an electric mixer, mix butter and powdered sugar on the lowest speed until smooth, about 5 minutes. Add vanilla and mix to combine.
2. Add two tablespoons of the cooled lemon curd and mix into the buttercream. Continue adding a tablespoon at a time until the buttercream is easy to stir and your spatula leaves soft peaks.
3. Fill a piping bag 3/4 full with the lemon 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. Spread a thin layer of lemon buttercream on top and then pipe a ring of buttercream around the edge to act as a dam or wall. Spoon lemon curd into the middle and spread it around to fill the buttercream dam.
2. Place your next layer of cake on top and repeat the filling: spread lemon buttercream, pipe a dam, and spoon lemon curd into the middle.
3. Place your final cake layer on top and then chill the cake for 30 minutes in the fridge or 15 minutes in the freezer.
4. Frost the cake with a thin layer of lemon buttercream (a crumb coat) and then a final layer.
5. Place the 8 raspberries you set aside earlier onto the top of the cake, arranging them opposite each other to space them evenly.
6. Spoon the rest of the lemon buttercream into a piping bag fitted with a 1M or 2D piping tip. Pipe a swirl or a ruffle in between each raspberry.