Make the dough: crumble the butter into the flour and sugar with your fingers.

Add egg and work into a dough ball and knead a couple minutes until smooth. Wrap in foil or plastic and refrigerate for at least 2 hours.

Prep the filling after the dough has chilled and preheat the oven to 190°C/375°F. Wash rhubarb, peel if desired (I'm too lazy but will get rid of any stringiness) and cut into 1/2 in (1-2 cm) pieces.

Grease a 9 in (22-23 cm) springform baking pan with butter. Roll out the dough into a large circle and line the pan with it, pressing the dough up the sides. If it falls apart in some parts, just press it in. Poke with a fork on the bottom.

In a the other bowl, beat the egg whites until it forms stiff peaks like a meringue. Quickly beat in the rest of the sugar.

Gently fold in the egg yolk and almond mixture.

Put the rhubarb into the springform pan evenly accross the crust dough. Pour over the egg mixture and even off the surface. Push down the sides of the dough if it's too high up the pan.

Bake in the 190°C/375°F oven for 40 minutes. Remove, and let cool for 10 minutes in the pan before removing the springform.

Once completely cooled, you can sprinkle with powdered sugar.