Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan or an 8x8 inch square baking pan.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt for the cake. Set aside.

In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. Beat in the egg and vanilla extract until well combined.

Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, starting and ending with the dry ingredients. Mix until just combined, being careful not to overmix. Gently fold in the chopped rhubarb.

Pour the cake batter into the prepared baking pan and spread evenly.

To make the streusel topping: In a separate small bowl, combine the flour, sugar, and cinnamon. Cut in the cold butter with a pastry blender or your fingertips until the mixture resembles coarse crumbs.

Evenly sprinkle the streusel topping over the rhubarb cake batter. Bake for 45-55 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cake cool in the pan on a wire rack for at least 15 minutes before carefully inverting it onto the rack to cool completely. Serve warm or at room temperature.