1stickbutter, softened, plus a bit more for the pan
1/4cuphalf and half
3-4large stalks rhubarb, in 1/2" slices (2 1/2 cups or 250 g)
For the streusel:
Combine the flour, sugar, cinnamon and salt in a medium bowl, or the bowl of a stand mixer. Add the butter, and work with your fingers or paddle on low until the mixture begins to clump together. Set aside at room temperature.
For the cake:
Position a rack in the center of the oven and preheat to 350º. Grease a 9 x 9 x 2" square pan generously with butter.
In the bowl of a stand mixer fitted with the paddle (or a bowl with a wooden spoon if you are badass) cream the butter and sugar together on medium until light and fluffy, 3 - 4 minutes, scraping down the sides of the bowl as necessary. Add the eggs one at a time, beating until combined after each, then stir in the vanilla.
While the wets are doing their thing, sift the dries into a medium bowl. Toss the rhubarb with the tablespoon of sugar and set aside. Stir together the yogurt and half and half.
With the mixer on low, alternate adding the dries and the dairy in three parts, beginning and ending with the dries and mixing until just combined after each addition. Give the batter a fold with a rubber spatula to make sure it is homogenous.
Spread a little more than half the batter in the bottom of the pan. Sprinkle the rhubarb evenly over the top. Cover with the rest of the batter, and sprinkle with the streusel, squeezing into almond sized clumps if necessary.
Bake the coffeecake for 50-60 minutes until it is golden brown, beginning to pull away from the sides, springs back to the touch, and passes the toothpick test. Let cool for about 1 hour. Cut into 16 squares.
The coffeecake is best the day it's baked when the streusel is crisp, but will keep for several days at room temperature, or in the fridge for up to a week (but I would warm it up in a toaster oven before eating).
If you forget to soften your butter ahead of time, no worries: cut it into small pieces and let it sit out while you get your other ingredients in place. If your butter is still chilly, you can make the streusel in a stand mixer with the paddle for ease, and warm the butter for the cake by placing the bowl over your oven while it preheats.For the dairy portion of the cake, I used a combination of yogurt and half and half, which I usually have around. I love the light texture I ended up with, but suspect the cake would work with buttermilk, milk, sour cream or any combination thereof.For a gluten-free and/or dairy-free variation, see this coffee cake recipe, swapping in rhubarb for the berries as directed. Nutritional values are based on one of sixteen squares.