Preheat your oven to 350°F (175°C).

Line a 9x9 inch baking pan with parchment paper or lightly grease it with non-stick spray.

Wash and chop 2 cups of fresh rhubarb into even pieces.

In a mixing bowl, combine the rhubarb with 3 tbsp of flour and 3 tbsp of powdered sugar. Toss until the rhubarb is evenly coated. Set aside.

Use a pastry cutter, fork, or your fingers to mix the ingredients together until they form a crumbly texture. Set aside.

In a large mixing bowl, sift or whisk together 2 cups of all-purpose flour, 1 tsp of baking soda, and 1 tsp of baking powder. Set aside.

In a separate mixing bowl, use an electric mixer to beat ½ cup of softened butter and ½ cup of granulated white sugar until light and fluffy, about 3-4 minutes.

Add the 3 room temperature eggs, one at a time, beating well after each addition.

Add the egg yolk and 1 tsp of vanilla extract, and continue to beat until well combined.

Gradually add the dry flour mixture to the wet ingredients, alternating with ½ cup of Greek yogurt or sour cream, beginning and ending with the flour mixture. Mix just until combined, being careful not to overmix.

Spread the cake batter evenly into the prepared baking pan.

Sprinkle the crumb topping evenly over the rhubarb layer.

Place the pan in the preheated oven and bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

Remove the cake from the oven and let it cool completely in the pan on a cooling rack.