Preheat oven to 150°C bake or 130°C fan bake. Arrange oven shelves in the centre and lowest racks.
Cut out two sheets of baking paper the size of large baking trays. Use a dark pen or pencil to trace six circles onto the paper. Find bowls/plates/cookie cutters that are roughly the following sizes to trace around:
1 x 17.5cm circle
1 x 9.5cm circle
1 x 4.5cm circle
1 x 15cm circle
1 x 12cm circle
1 x 7cm circle
Leave a few centimetres between each circle (though pavlova should not expand very much). Flip the pieces of baking paper over and place on baking trays.
Place egg whites in the bowl of a stand mixer and beat on high speed until soft peaks form (1 1/2 minutes). Continue to beat while adding Chelsea Caster Sugar a tablespoon at a time (1 1/2 minutes).
Beat for a further 7-8 minutes, until mixture is thick and glossy, and no longer grainy (the mixture should be smooth when you rub it between your fingers).
Sift in Edmonds Fielder's Cornflour and add vinegar. Beat until combined (about 30 seconds). The mixture should be stiff enough at this point that you can flip the bowl upside down without it falling out.
Transfer mixture to a large piping bag fitted with a large open star piping tip (you may need to do this in two batches, depending on the size of your piping bag). Pipe tight coils of mixture within the traced out areas on the baking paper. Pipe a second coil on top, so that each round is 2-3cm thick.
Place trays in oven, then immediately turn temperature down to 120°C bake or 100°C fan bake. Bake for 60-65 minutes. The bases of the pavlova rounds should be dry - open the oven door slowly and carefully check this while they are in the oven if you can, but do not remove from oven!
Turn oven off, prop the oven door ajar slightly and allow pavlova to cool completely in oven (about 3 hours). Once cool, transfer to airtight containers until ready to assemble (use baking paper between layers).
Place raspberries, Chelsea Caster Sugar, chia seeds (if using) and lemon juice in a small-medium saucepan on medium heat and simmer, stirring occasionally, until thickened (10-15 minutes). Remove from heat and leave to cool. Refrigerate until required.
Pavlova tree should be assembled just before serving, and ideally in or near the place it will be served.
Just before you are ready to assemble the tree, hull and thinly slice strawberries.
Place cream, Chelsea Icing Sugar and vanilla extract in a large bowl and beat until stiffly whipped. Transfer to a large piping bag fitted with a large open star piping tip. Piping is better than spreading the cream with a spoon or knife, as it is more gentle on the fragile pavlova.
Pipe a little cream onto the base of a large serving board or plate. Secure the largest pavlova round in the middle of the board/plate. Pipe a coil of cream on top. Add some of the sliced strawberries and blueberries, then drizzle with raspberry compote. Pipe a small circle of cream in the middle, to help glue the next layer on. Repeat with the next four layers, before adding the final pavlova piece on top.
If the tree starts to lean at any point, insert a large skewer through the middle to help secure everything in place.
Insert the star on the top, if using. Dot raspberries around the edges of the tree, then decorate with small rosemary sprigs. Dust the whole tree with Chelsea Icing Sugar. Serve immediately!