Heat the milk in a saucepan until just steaming, then remove from the heat and stir in the sugar and butter, swirling until the butter melts. Tip the flours and yeast into a bowl and season with 1/4 tsp salt. Make a well in the middle and pour in the milk mixture and the egg yolks. Whisk until you have a smooth batter, then cover with cling film and set aside for 1 hr (if you want to make the blinis ahead and freeze them, jump to step 4).
Meanwhile, prepare the toppings. Bring a large pan of water to the boil, add the quail’s eggs and cook for 2 1/2 mins. Drain the eggs and run under cold water, then leave to cool completely. Tip the beetroot and cream cheese into a food processor and season with a pinch of salt and a generous grind of black pepper. Whizz to a purée, then scrape into a bowl and chill until ready to serve.
For the salmon topping, flake the salmon into a bowl, add the cream cheese, lemon zest and juice, and horseradish, if using. Mash everything together with a fork, then chill until ready to serve. Peel the cucumber into long strips using a vegetable peeler, discarding the seedy middle part, and pop into a bowl of cold water.
Check the blini batter – it should be bubbly on the surface. Tip the egg whites into a bowl and whisk until they hold soft peaks. Add to the blini batter and gently fold together, trying to keep as much air in the mixture as possible.
Get a wire rack or two ready to cool the blinis (this will prevent them from going soggy when cooling). Heat a knob of butter and a drizzle of oil in a large pan (if you have two pans, this will speed things up). When the butter is foaming, spoon tablespoons of the batter into the pan, leaving space between each one as they will spread a little. Cook over a medium heat until bubbles pop on the surface and the undersides are golden brown. Flip over and cook for 1-2 mins more, then transfer to the rack to cool. Wipe out the pan and continue cooking the remaining blinis in the same way. Will keep in an airtight container for 1 day, or freeze the cooked blinis on a baking tray covered with cling film, then foil, for up to a month. Defrost at room temperature before topping.
To assemble, top half the blinis with the beetroot purée, peel and halve the eggs and place them on top, then scatter with cress and season with a little black pepper. Top the remaining blinis with the salmon pâté. Dry the cucumber slices well and ruffle each piece back on itself, then squash firmly between your fingers so it lays flat. Place them on top of the salmon, add a tiny blob of caviar and eat as soon as possible, preferably with glasses of fizz!