Preheat your oven to 350°F and spray a 9x9-inch cake pan with nonstick spray.

In a large bowl, beat together 2 tablespoons of softened unsalted butter, 1 cup of granulated sugar, 1 large egg, and 1 teaspoon of pure vanilla extract until light and fluffy.

In a separate bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon of baking powder, 1/2 teaspoon of baking soda, and 1/2 teaspoon of fine sea salt.

With the mixer on low, incorporate half of the dry mixture into the butter and sugar mix. Then add 1 cup of buttermilk and mix until smooth, followed by the remaining dry mixture.

Gently fold in 3 cups of chopped rhubarb.

Spread the batter evenly into the prepared cake pan.

In a small bowl, mix together 1/2 cup of all-purpose flour, 1/2 cup of lightly packed light brown sugar, and 4 tablespoons of melted unsalted butter until clumps form.

Sprinkle the streusel evenly over the top of the cake batter.

Bake in the preheated oven for 60-70 minutes or until a toothpick inserted into the center comes out clean.

Let the cake cool in the pan for a little while before transferring it to a wire rack to cool completely.