This is, hands-down, my favorite coffee cake, like, ever. I love how plums, when they’re baked into batter, soften and leach their gorgeous purple color (either from the skin, flesh, or both) right into the crumb. Sweet-tart, they add moisture and variation to this tasty, tender slab cake. The streusel is crumbly and buttery and, to me, perfect. If you’re not into walnuts, use any nut you like. You can also make the cake with raspberries, blueberries, apricots, blackberries, or ripe pears.
Ingredients
FOR THE WALNUT STREUSEL
- 1 cup finely chopped walnuts
- 1 cup all-purpose flour
- ¼ cup packed light brown sugar
- ¼ cup sugar
- ½ teaspoon kosher salt
- ½ cup (1 stick) cold unsalted butter, cut into very small pieces
FOR THE PLUM CAKE
- 2½ cups all-purpose flour
- 1 teaspoon baking powder
- ½ teaspoon kosher salt
- ¼ teaspoon baking soda
- 3 large eggs
- ¾ cup sugar
- 2 teaspoons pure vanilla extract
- 1 cup sour cream
- 1 cup (2 sticks) unsalted butter, melted
- 4 plums or pluots, halved, pitted, and roughly chopped (about 1½ cups)
How to Make It
- Adjust an oven rack to the middle position and preheat the oven to 350°F. Grease the bottom of a 10 x 16-inch rimmed sheet pan with 1 tablespoon room-temperature butter. Line the pan with parchment paper (you may need 2 sheets) so the bottom and sides are covered and there is enough paper sticking above the rim to help remove the cake from the pan for serving. Press the paper into the pan, then flip it, so both sides are greased, and press it back into the pan. Set aside.
- To make the streusel: Combine the walnuts, flour, brown sugar, sugar, and salt in a medium-size bowl. Add the butter and, with your fingers, work it in until the mixture is very fine and crumbly and no butter pieces larger than a small pea remain. Set aside.
- To make the cake: Whisk the flour, baking powder, salt, and baking soda in a medium-size bowl. Whisk the eggs, sugar, and vanilla in a large bowl until foamy and creamy, about 2 minutes (or use an electric mixer if you like). Whisk in the sour cream, then add the flour mixture, whisking until just a streak or two of dry ingredients remains. Pour in the melted butter and whisk until just combined.
- Pour the batter into the prepared sheet pan and sprinkle the plums over the top, followed by the streusel. Bake until the cake is golden brown and resists light pressure, 25 to 30 minutes. Remove from the oven and set aside to cool completely. Use the parchment paper to help lift the cake from the pan, slice, and serve. The cake will keep in an airtight container at room temperature for 3 to 4 days.
