Preheat your oven to 375°F (190°C). Grease and flour an 8x8 inch baking pan. Alternatively, you can line it with parchment paper.

**For the Cake Base:** In a large bowl, whisk together the 1 1/2 cups flour, 1 cup sugar, baking powder, and salt.

In a separate medium bowl, cream together the softened 1/2 cup butter and egg until light and fluffy. Gradually add the milk and mix until just combined.

Add the wet ingredients to the dry ingredients, mixing until just combined. Do not overmix. Gently fold in the chopped rhubarb. Pour the batter into the prepared baking pan, spreading evenly.

**For the Streusel Topping:** In a small bowl, combine the 1/2 cup flour, 1/2 cup sugar, and ground cinnamon. Cut in the cold 1/4 cup butter using a fork or your fingertips until the mixture resembles coarse crumbs.

Sprinkle the streusel topping evenly over the rhubarb batter. Bake for 45-55 minutes, or until a wooden skewer inserted into the center comes out clean and the topping is golden brown. Let cool on a wire rack before serving.