This started innocently enough, with a delicious-looking lemon blueberry bundt cake. The blueberries were basically sandwiched in between two layers of batter, and the finished cake was covered with a glaze. I foresaw a problem immediately, but since the author of that recipe didn’t encounter it, I thought I must be wrong, but it turns out I wasn’t. See, unmolding a cake which has a middle layer of fruit means that the cake will separate in the middle, with what should be the upper layer stuck in the pan. I did manage to get it out, but lost a few pieces on top, so I used the glaze to hide the unfortunate unmolding. It was a good cake, but I found it so frustrating that I had no interest in making it again. (I also had unmolding issues with this summer fruit bundt pound cake.)
Then I came across Cook’s Illustrated’s recipe for marbled blueberry bundt cake, and this one is a winner. Mixing the blueberries in the batter entirely solves the above problem, and blending them with fruit pectin avoids pockets of fruit that burst (this can be great in some desserts, but in a bundt cake, not so much). Plus, it unmolded beautifully!
I recommend starting with the filling (even though the original recipe has one make it after the cake batter). The recipe calls for fruit pectin such as Sure-Jell (Ball will not work). I couldn’t find any, so I ended up using a cornstarch slurry to thicken my blueberry purée. This means it was probably cloudier than what was desired, but I didn’t mind. I assembled the batter while the filling cooled.
For the filling
¾ cup (5¼ oz.) sugar
3 Tbsp. low- or no-sugar-needed fruit pectin (see note above)
1 pinch salt
10 oz. (2 cups) fresh or thawed frozen blueberries
1 tsp. grated lemon zest plus 1 Tbsp. juice
For the cake
3 cups (15 oz.) all-purpose flour
1½ tsp. baking powder
¾ tsp. baking soda
1 tsp. salt
½ tsp. ground cinnamon
¾ cup buttermilk (lactose-free milk with a splash of lemon juice)
2 tsp. grated lemon zest plus 3 tablespoons juice
2 tsp. vanilla extract
3 large eggs plus 1 large yolk, at room temperature
18 Tbsp. (2 ¼ sticks) unsalted margarine, softened
2 cups (14 oz.) sugar
For the filling
Whisk sugar, pectin, and salt together in small saucepan. Process blueberries in blender until mostly smooth, about 1 minute. Transfer ¼ cup purée and lemon zest to saucepan with sugar mixture and stir to thoroughly combine. Heat sugar-blueberry mixture over medium heat until just simmering, about 3 minutes, stirring frequently to dissolve sugar and pectin. Transfer mixture to medium bowl and let cool for 5 minutes. Add remaining purée and lemon juice to cooled mixture and whisk to combine. Let sit until slightly set, about 8 minutes.
For the cake
Adjust oven rack to lower-middle position and heat oven to 325 °F. Heavily spray 12-cup nonstick Bundt pan with baking spray with flour. (If you don’t have nonstick baking spray with flour, mix 1 tablespoon melted butter and 1 tablespoon flour into a paste and brush inside the pan.)
Whisk flour, baking powder, baking soda, salt, and cinnamon together in large bowl. Whisk buttermilk, lemon zest and juice, and vanilla together in medium bowl. Gently whisk eggs and yolk to combine in third bowl.
Using stand mixer fitted with paddle, beat butter and sugar on medium-high speed until pale and fluffy, about 3 minutes, scraping down bowl as needed. Reduce speed to medium and beat in half of eggs until incorporated, about 15 seconds. Repeat with remaining eggs, scraping down bowl after incorporating. Reduce speed to low and add one-third of flour mixture, followed by half of buttermilk mixture, mixing until just incorporated after each addition, about 5 seconds. Repeat using half of remaining flour mixture and all of remaining buttermilk mixture. Scrape down bowl, add remaining flour mixture, and mix at medium-low speed until batter is thoroughly combined, about 15 seconds. Remove bowl from mixer and fold batter once or twice with rubber spatula to incorporate any remaining flour. Cover bowl with plastic wrap and set aside while preparing filling; batter will inflate a bit. (If you made the filling beforehand, like me, it’s easy to just wait a bit here. But if you’re struggling to get the right consistency with your blueberry filling while worrying about your batter being too far gone now that you’ve mixed wet and dry ingredients, it’ll be nerve-wracking!)
Spoon half of batter into prepared pan and smooth top. Using back of spoon, create ½-inch-deep channel in center of batter. Spoon half of filling into channel. Using butter knife or small offset spatula, thoroughly swirl filling into batter (there should be no large pockets of filling remaining). Repeat swirling step with remaining batter and filling.
Bake until top is golden brown and skewer inserted in center comes out with no crumbs attached, 60 to 70 minutes. Let cake cool in pan on wire rack for 10 minutes, then invert cake directly onto wire rack. Let cake cool for at least 3 hours before serving.