• Preheat the oven to 350°F (180°C). Grease or line a 9 x 13-inch baking pan with parchment paper.
• Make the streusel. In a bowl, combine the flour, brown sugar, cinnamon, and salt. Add the room temperature butter and use your fingers to crumble it into the dry ingredients until you have a mix of fine crumbs and larger, cookie-dough-like pieces. Toss in the chopped walnuts and set aside. 60 g all-purpose flour, 50 g soft brown sugar, 2 tsp ground cinnamon, 1/4 tsp salt, 60 g unsalted butter, 50 g walnuts
• Chop the rhubarb into 1/4-inch (1/2 cm) pieces and set aside.

Cake batter

• Whisk together the flour, baking powder, baking soda, and salt in a bowl. Set aside. 300 g all-purpose flour, 2 tsp baking powder, ¾ tsp salt, ¼ tsp baking soda
• In the bowl of a stand mixer fitted with the paddle attachment (or use a handheld electric mixer), beat the butter and sugar on medium speed until light and creamy. Scrape down the sides as needed. 170 g unsalted butter, 200 g granulated sugar
• Beat in the eggs one at a time, followed by the vanilla. The batter may look curdled at this point, that's normal. 3 large eggs, 2 tsp vanilla paste
• Add half the dry ingredients and half the yogurt and fold together gently with a spatula. Add the remaining dry ingredients and yogurt and fold until just combined. Don't over-mix. 240 g natural yogurt
• Spread the batter into the prepared pan using an offset spatula or the back of a spoon.
• Scatter the chopped rhubarb evenly over the batter, then pile the streusel topping on top. 300 g rhubarb
• Bake for 35-40 minutes until the streusel is golden and a skewer inserted into the center comes out with a few moist crumbs (but no wet batter).
• Let the cake cool in the pan for at least 10 minutes before slicing.