Heat oven to 200C/180C fan/gas 6. Butter the sides of two 20cm round cake tins and line the bases with baking parchment.
To roast the rhubarb, line a high-sided baking tray with baking parchment, add the rhubarb, then top with the sugar, lemon juice, zest and cordial. Cover with foil and roast in the oven for 25 mins until soft. Remove from the oven and reduce the temperature to 190C/170C fan/gas 5. Strain the rhubarb from the poaching liquor, reserving the syrup.
Tip the almonds into the bowl of a food processor and blitz until ground (if using whole almonds), but still retaining some texture.
Put the butter in the bowl of a stand mixer fitted with a whisk attachment (or use an electric whisk) and cream it on medium speed for 2 mins, then, using a rubber spatula, scrape down the butter from the sides of the bowl, then tip in the sugar and lemon zest. Turn up the mixer speed slightly and continue whisking for 5 mins until light and fluffy. Turn the mixer speed down slightly, then add the eggs, egg white, vanilla extract and cordial and mix together.
Combine the flour, almonds, baking powder, bicarbonate of soda and a pinch of salt in a bowl. Scrape the butter mixture down from the sides of the stand mixer bowl, turn the mixer to low speed – so it’s just stirring – then spoon in the flour and almond mixture in four batches, alternating with spoonfuls of the yogurt, and finishing with a batch of the flour and almond mixture. Mix until just combined, with no streaks remaining, then add half the roasted rhubarb and 1 tbsp of its syrup to the batter and fold it in.
Divide the batter evenly between the cake tins and bake in the oven for 25-30 mins until a skewer inserted into each cake comes out clean. Remove the cakes from the oven, leave them to cool completely in the tins on a wire rack, then carefully remove from the tins.
For the filling, whip the cream to soft peaks in a bowl and fold in three-quarters of the remaining roasted rhubarb, along with the elderflower cordial and yogurt.
Put one of the cakes flat-side down on a cake stand or plate, prick it all over with a skewer, and use a pastry brush to brush the cake with a little elderflower cordial. Spoon on half the cream mixture, top with the other cake, and brush that cake with cordial, too. Spoon over the remaining cream mixture, top with the remaining rhubarb and spoon over some of the poaching syrup. Garnish with elderflowers or seasonal flowers.