For the pastry, measure 75ml just-warm water, add the yeast and stir it to dissolve. Measure another 75ml ice-cold water and add the lemon juice to it. Mix the flour and sugar with 1/2 tsp fine salt in a large bowl, then toss in the cubes of butter until coated in the flour. Splash the yeast water and lemon water over the contents of the bowl. Using a round-bladed knife, work quickly to bring the mix to a rough dough with lumps of butter held inside it.
Turn onto a floured work surface, shape into a squat rectangle without kneading it too much, then wrap in cling film and chill for 15 mins in the freezer.
Dust the work surface and pastry with flour. Roll the pastry in one direction until it’s 3 times long as it is wide, or about 45 x 15cm. Try to keep the sides straight as you roll, and the top and bottom edges as square as possible.
Fold the pastry over itself. Fold the bottom third up, then the top third down, to make a neat block. Turn the block so that its open edge is facing to the right, like a book. Press the edges together gently with the rolling pin. Roll out and fold the pastry like this 3 more times to make a smooth dough, with the odd streak of butter here and there. If it feels greasy at any point, or becomes springy, cover and chill for 10 mins before continuing. Wrap the finished dough and chill for at least 1 hr.
To make the filling, beat together all the ingredients and season with a pinch of salt. Set aside until needed.
To shape, put the dough on a floured work surface. Cut it in half across the middle and return one half to the fridge. Roll the other to a 30 x 20cm rectangle, then cut this into 3 strips, each 10cm wide. To create even more layers, fold each piece in half lengthways. Put a generous teaspoon of the almond mix at one end of the pastry, then roll the pastry around it in a spiral. Place, cut-edge up, into a non-stick muffin tin. Repeat to make 6 cruffins. Loosely cover with cling film. Can be chilled overnight. (You may have some filling left over, which can be frozen.)
Prove the dough at a cool room temperature for 2 hrs or until the pastry has filled the wells of the tin. Heat oven to 190C/170C fan/gas 5. Bake the cruffins for 20-25 mins until risen and deep golden brown. Remove and cool a little on a rack. To fill with jam, spoon the jam into a piping bag fitted with a 5mm nozzle, or use a chef’s squeezy bottle. Push the nozzle into the middle of the cruffin and squeeze. Dust the cruffins with icing sugar and eat cool or warm on the day of baking.