Preheat oven to 175°C. Peel and chop the rhubarb. Prepare 1 (or 3 - 4) 16 cm baking pan.
Mix butter and sugar until fluffy. Add Eggs one after one. Mix flour and baking powder and sift it. After all eggs are incorporated, add salt, vanilla extract and flour mix until dough is smooth. Fold in rhubarb. Weigh the dough and divide it into 3 - 4 equal portions. Bake each portion successively if you have one baking pan or at the same time if you have more bakin pans for 15 - 20 minutes. Let the cake layers cool down on a wired rack.
Buttercream: Heat egg whites and sugar over a bain marie, stir until sugar is completly dissolved. Once the sugar is dissolved, mix until its stiff and white. The bowl should be cold now (up to 10 minutes). Meanwhile melt chocolate over a bain marie, chop rosemary and add it to the chocolate. As soon as the meringue is ready, mix in soft butter, piece by piece. Ad the end, fold in the white chocolate. Keep in fridge until you can assembly the cake.
Cake assembly: As soon as the cake layers are chilled you can assembly the cake. Cut strawberries in stripes. Add 1/3 of the buttercream on to the first cake layer. Spread some strawberry stripes on the buttercream and add the second cake layer on top. Spread 1/3 of the buttercream and add the third cake layer on top - if you have four cake layers spread only 1/4 of the buttercream between each layer. Spread the remaining buttercream on the last layer and decorate with the remaining strawberries and some rosemary sprigs if you like. Keep in fridge until you eat it.