Rhubarb’s bright flavor, transformed into sweet-tart syrup, infuses this coffee cake. Topped with warm, gingery streusel, the tender cake is a bite of springtime.
To prepare the rhubarb: In a medium bowl, stir together the diced rhubarb and sugar. Pour the mixture into a strainer and place the strainer over a bowl. Let the rhubarb drain for 45 minutes, tossing occasionally; reserve the rhubarb and liquid separately.
To make the streusel: In a medium bowl, whisk together the sugar, flour, cinnamon, ginger, and salt. With a fork or your fingers, work in the butter until the mixture is crumbly, then stir in the nuts. Set aside.
Preheat the oven to 350°F. Lightly grease a 9" x 13" pan.
To make the cake: In a large bowl, whisk together the flour, baking powder, salt, and sugar.
Add the butter, stirring or beating gently until the mixture looks sandy.
Mix in the rhubarb juice, vanilla, and lemon zest, then beat in the egg. Scrape the bowl and mix for 30 seconds; the batter will be thick.
Stir in the drained rhubarb by hand, mixing until evenly distributed.
Transfer the batter to the prepared pan and sprinkle the top with the streusel.
To bake the cake: Bake the cake for 47 to 50 minutes, until a paring knife inserted into the center comes out clean. Remove the cake from the oven and cool it on a rack before serving.
Storage information: Store the cake, well wrapped, at room temperature for up to four days; freeze for longer storage.
