Grease the baking tin with butter.
Peel rhubarb and cut it into cubes.
Beat soft butter, sugar, vanilla and salt for about 5 minutes until creamy.
Add vegetable oil and beat briefly until homogeneous.
Add eggs one by one, beating each time until homogeneous.
Mix flour with baking powder.
Add the flour mixture to the egg-butter mixture and mix briefly to form a thick batter.
Pour the batter into the baking tin and smooth it out.
Spread the rhubarb cubes on the batter, pressing some pieces in a bit.
Bake the cake in a preheated oven at 356 °F (180 °C) for about 40 minutes.
Let the rhubarb cake cool a bit in the baking tin first and then take it out.
Dust the rhubarb cake with powdered sugar before serving.