Heat oven to 350°F. Prepare an 8-9 inch spring form pan by greasing the bottom and the sides and lining the bottom with parchment paper and greasing it as well.
Cream the butter and the sugar with an electric mixer, then add the egg yolks and mix until well incorporated. Add the vanilla.
Mix in the flour, baking powder, and salt, beating the batter until it is smooth.
Add the milk and continue beating to incorporate.
Put the batter in the spring form pan and cover the top with the chopped rhubarb.
Bake for 45 minutes.
About 7 minutes before the cake is done, start beating the 4 egg whites on high. Gradually sprinkle in 1 cup (230 grams) of sugar. Beat until very stiff, 7-10 minutes, depending on the power of your mixer.
Take the cake out of the oven, spoon the meringue on top, and spread it out evenly.
Put the cake back in the oven for 20 more minutes.
When the cake is done, you can remove it to a wire rack to cool in the pan, or you can leave it in the oven, turn the oven off, and leave the door open slightly while it cools.
Cool the cake completely, at least to room temperature, before eating. It's delicious cold from the fridge, too!
Bon Appetit!