Preheat your oven to 180°C (350°F). Grease and flour your chosen cake tin (a ring tin or square tin both work well).

In a large bowl, cream together the brown sugar and butter until light and fluffy.

Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

Sift in the flour and baking powder, then gently fold into the wet mixture.

Stir the sour milk into the batter until combined.

Fold in the chopped rhubarb.

Pour the batter into your prepared cake tin and spread evenly.

In a small bowl, mix the brown sugar and cinnamon for the topping. Sprinkle evenly over the batter.

Bake in the preheated oven for 45 minutes, or until a skewer inserted into the center comes out clean.

Allow the cake to cool in the tin for 10 minutes before transferring to a wire rack. Serve warm or at room temperature, on its own or with a dollop of cream.