Prepare the Streusel: In a mixing bowl, whisk together flour, sugar, cinnamon, and salt until well combined. Cut in the cold butter until the mixture resembles coarse crumbs. Refrigerate while proceeding to next steps.

Preheat the Oven: Preheat your oven to 360°F (180°C) and prepare a 9x13-inch baking pan by lining it with parchment paper or greasing it with butter.

Cream the Butter and Sugar: In a large mixing bowl, add the softened unsalted butter and granulated sugar, then beat until light and fluffy.

Incorporate the Eggs: Add the eggs one at a time, mixing on low speed until fully incorporated.

Mix in Wet Ingredients: Add plain Greek yogurt and vanilla extract to the egg mixture, and stir gently until just combined.

Combine Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, ground cinnamon, and salt. Gradually fold into the wet ingredients.

Assemble the Cake: Spread the batter into the prepared pan, layer with rhubarb and sprinkle streusel on top.

Bake the Cake: Bake in the preheated oven for 50-60 minutes, checking for doneness with a toothpick.

Cool and Serve: Allow to cool in the pan for 15-20 minutes before serving.