Preheat your oven to 350°F (175°C). Grease and flour an 8x8 inch square baking pan or line with parchment paper.

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

In a large bowl, using an electric mixer, cream together the softened butter and granulated sugar until light and fluffy (about 3 minutes).

Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Mix in the vanilla extract.

Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix only until just combined; do not overmix.

Gently fold in the chopped rhubarb pieces until evenly distributed throughout the batter.

Pour the batter into the prepared pan and spread evenly. If using the topping, mix the brown sugar and cinnamon, and sprinkle evenly over the top of the batter.

Bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan on a wire rack for at least 20 minutes before slicing and serving warm or at room temperature.