Prep: Line a 9×13 inch (22 x 33cm) cake pan and preheat the oven to 360°F (180°C).
Make the streusel: Add flour, sugar, ground cinnamon and salt for the streusel to a small bowl. Mix well, then cut in the cold butter until crumbs form. Chill in the fridge until ready to use.
Cream wet ingredients: Cream butter, sugar and eggs together until light and fluffy (this takes 4-5 minutes, so be patient or your cake won't rise). Slowly beat in greek yogurt and vanilla extract.
Combine dry ingredients and make cake batter: Combine flour, baking powder, ground cinnamon and salt for cake in a separate bowl. Slowly mix into the egg mixture until a smooth batter forms – it will be thick.
Assemble and bake the cake: Spread the batter in the prepared cake pan. Arrange the sliced rhubarb on top, slightly pressing it into the batter. Scatter the streusel over the cake. Bake the cake for 50-60 minutes, until a toothpick inserted into the middle comes out fairly clean. Cool in the pan on a wire rack before slicing to serve.
Chilling the streusel helps it to stay in thick, cookie-like crumbs during baking. So don't skip this step! It only needs to chill while you're making the rest of the cake, so it won't add any additional waiting time.This makes a huge amount of streusel. If you prefer less streusel, cut the amount in half (for reference, I took the photos with half the amount, to make sure the rhubarb shows well in the pictures).Do not skip beating the eggs until fluffy. This really does take around 4 minutes - otherwise the cake will be dense and flat.The batter is fairly thick, don't be alarmed by this. It needs to be sturdy to support the load of fruit on top. If it seems impossibly thick, add a couple tablespoons of milk.
Fruit: This cake works with many different kinds of fruit. Peaches, plums, strawberries, raspberries, apples... Change it up or use a mix, if you like! The juicier the fruit, the moister the cake will be.Greek yogurt: Sour cream can be used in place of the greek yogurt. Plain yogurt can be used, but you'll need to increase the flour by a couple of tablespoons.Butter: The cake really works best with real butter. Especially the streusel turns out really well with it. If you don't have butter or want to make the cake more budget friendly, stick margarine can be substituted (it will slightly change the flavor and texture of the cake and streusel). Do make sure to use stick margarine and not a spread!Vanilla extract: A delicious twist on the cake is using 1 teaspoon vanilla extract and 1 teaspoon almond extract in place of all vanilla in the batter.
The cake keeps at room temperature for up to 2 days. Cover it well to keep it from drying out.