Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan with butter or cooking spray, then dust with flour.

In a large mixing bowl, cream together 1 cup of softened unsalted butter and 1 ½ cups of granulated sugar until light and fluffy, about 3–4 minutes.

Beat in 3 large eggs one at a time, then pour in 1 cup of whole milk, mixing until well combined.

In a separate bowl, whisk together 2 cups of all-purpose flour, 2 teaspoons of baking powder, and a pinch of salt.

Gradually add the dry mixture to the wet batter while mixing gently. Fold in 2 cups of chopped fresh rhubarb with a spatula.

Pour the batter into the greased cake pan, smoothing the top.

In a small bowl, mix ¼ cup of granulated sugar and 1 teaspoon of ground cinnamon. Sprinkle this over the batter.

Bake in the preheated oven for 40–45 minutes, or until a toothpick inserted comes out clean.

Allow the cake to cool in the pan for 20 minutes before transferring to a wire rack to cool completely.