Preheat your oven to 180°C (350°F). Grease and flour a 9-inch (23 cm) springform pan or line a square baking dish.

Prepare the batter: Cream the softened butter and granulated sugar together until light and fluffy. Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.

In a separate bowl, whisk together the flour and baking powder. Gradually add the dry ingredients to the wet ingredients, alternating with the milk, mixing only until just combined. Do not overmix.

Prepare the crumble: Combine the flour, brown sugar, and cinnamon for the topping in a small bowl. Cut in the cold cubed butter using your fingertips or a pastry blender until the mixture resembles coarse crumbs.

Spread half of the batter evenly into the prepared pan. Sprinkle about two-thirds of the chopped rhubarb over the batter. Top with the remaining batter, then sprinkle the rest of the rhubarb on top.

Evenly distribute the prepared crumble topping over the rhubarb layer. Bake for 45 to 50 minutes, or until a toothpick inserted into the center (avoiding rhubarb) comes out clean.

Let the cake cool in the pan for 15 minutes before carefully releasing the sides. Cool completely on a wire rack before serving. Dust with powdered sugar if desired.