Melt the butter for the topping.

Line the inside of a springform pan with baking paper.

Pour the melted butter into the springform pan and sprinkle the brown sugar on top.

Rinse the rhubarb and cut them into thin slices, possibly at a slight angle.

Place the rhubarb on top of the brown sugar in the springform pan.

Mix the soft butter and sugar together in a bowl.

Add one egg at a time while stirring.

Grate a bit of lemon zest and squeeze the lemon for 2 tablespoons of juice.

Add the lemon juice, lemon zest, and vanilla sugar to the butter and sugar mixture and make sure it is all well combined.

Pour the batter over the rhubarb in the springform pan.

Place the cake in a preheated oven at 175 degrees Celsius (347 degrees Fahrenheit) (conventional oven) and let it bake for about 45 minutes. The baking time depends on your oven, so it may vary by about 5 minutes on either side.

Take the cake out of the oven and turn it out onto a cake platter.