• Preheat oven to 350°F and grease or line an 8x8-inch or 9x9-inch square baking pan with parchment paper; set aside.

Make the Streusel Topping

• In a small bowl, combine the brown sugar, cinnamon and flour.
• Add the melted butter and stir until combined; refrigerate until ready to use.

Make the Batter

• Add the flour, baking powder, baking soda and salt to a bowl; stir and set aside.
• Add the butter and sugars to a large mixing bowl or a the bowl of a stand mixer.
• Cream the butter and sugars for a couple minutes, or until light and fluffy.
• Add the eggs and vanilla; stir well, scraping down the sides at least once.
• Add about half of the dry ingredients and mix to combine.
• Continue mixing while adding the buttermilk slowly; scrape the sides of the bowl.
• Add the remaining dry ingredients, stirring until just combined.

Assemble and Bake

• Pour a little more than half of the batter into the prepared pan; spread to an even layer.
• Sprinkle the rhubarb over the batter and then about 1/2 cup of the streusel topping.
• Spoon the remaining batter on and smooth it out to an even layer.
• Top with the streusel topping, pressing it into the batter gently.
• Bake for about 30 minutes, or until a toothpick inserted in the center comes out with just a few moist crumbs (not runny batter). If the top is browning but the cake isn't done in the center, top with aluminum foil and continue to bake. This will keep it from over-browning.
• Let cool, then cut and enjoy!