Preheat the oven to 180 degrees Celsius/ 350 degrees Fahrenheit. Line a baking tray (about 30x40 cm/ 12x16 inches) with baking paper.
Rhubarb: clean and chop into pieces about 1.5-2 cm/ 0.6-1-inch. Set it aside.
Crumb topping: mix sugar, flour, and salt in a bowl. Cut cold butter into small pieces and add them to the bowl. Add a little vanilla if desired. Work in with the handheld mixer fitted with the dough attachment or with your cold hands. Place topping in the fridge while you make the batter.
Batter: Beat soft butter, sugar, and salt until light and fluffy. Add eggs, one at a time, beating well in between, about 1-2 minutes beating time per egg. Stir in the vanilla extract. Mix flour and baking powder. Add to the batter together with the yogurt and mix shortly.
Bake: Spread the batter on the prepared baking tray. Cover with rhubarb and top with crumbs. Bake for 40-45 minutes until the crumbs are golden brown and slightly crunchy