Blueberry Buckle

Before you start

A buckle is an old American cake where the fruit outweighs the batter. So many blueberries go into a thick, buttery batter that the cake rises around them unevenly, and the crumb topping sinks into the low spots as it bakes, which gives the surface its buckled look.

Because there is so much fruit, the batter must be stiff enough to hold the berries in suspension, and the cake needs a long, moderate bake. Pull it too early and the middle is gummy with berry juice. Test with a skewer between berries rather than through one; a few moist crumbs are fine, wet batter is not.

Ingredients

• Crumb topping65 g (½ cup) all-purpose flour
• 70 g (⅓ cup packed) light brown sugar
• 1 tsp ground cinnamon
• Pinch fine salt
• 55 g (4 tbsp) unsalted butter — melted
• Cake250 g (2 cups, spooned and leveled) all-purpose flour — plus 1 tbsp for the berries
• 2 tsp baking powder
• ½ tsp fine salt
• 115 g (½ cup) unsalted butter — softened
• 150 g (¾ cup) granulated sugar
• 1 large egg — room temperature
• 2 tsp vanilla extract
• 1 tsp finely grated lemon zest
• 120 ml (½ cup) whole milk — room temperature
• 450 g (3 cups) blueberries — fresh, or frozen and not thawed

Method

1. Heat the oven to 180°C (350°F). Grease the pan and line it with a parchment sling.
2. Make the crumb topping: stir the flour, brown sugar, cinnamon and salt, then add the melted butter and mix with a fork until it forms clumps. Refrigerate while you make the batter.
3. Whisk 250 g flour, the baking powder and salt in a bowl.
4. Beat the softened butter and sugar on medium speed for 2 to 3 minutes, until pale. Beat in the egg, vanilla and lemon zest.
5. On low speed, add the flour mixture in three parts, alternating with the milk in two parts, and mix just until combined. The batter will be thick, like soft cookie dough.
6. Toss the blueberries with 1 tablespoon of flour and fold them into the batter with a spatula. Spread it in the pan, then scatter the crumb topping evenly, squeezing some into larger clumps.
7. Bake for 50 to 60 minutes, until the top is golden, berries are bursting at the surface and a skewer inserted between berries in the center comes out with no wet batter.
8. Cool in the pan on a rack for at least 30 minutes, then lift out with the sling and cut into squares.

Tips for success

• A metal pan bakes the dense center more evenly than glass. In a glass dish, lower the oven to 165°C (325°F) and expect a longer bake.
• Frozen berries go straight from the freezer into the batter. Thawed ones bleed and turn the batter gray-blue.
• Smaller wild blueberries distribute more evenly; use the same weight.

Substitutions

Whole milk

Buttermilk or thinned plain yogurt gives a slightly more tender, tangy crumb. Add ¼ tsp baking soda with the dry ingredients.

Blueberries

Raspberries, blackberries or diced peaches work at the same weight. Peaches release more juice, so add 5 to 10 minutes.

All-purpose flour in the cake

Replace up to 60 g with whole wheat pastry flour for a nuttier, slightly denser cake.

Variations

Lemon glaze

Drizzle the cooled cake with 60 g powdered sugar whisked with 1 tbsp lemon juice.

Oat crumb

Replace half the flour in the topping with old-fashioned rolled oats for a crunchier finish.

Troubleshooting

Gummy center

Underbaked. Bake until a skewer between berries shows no raw batter, and do not cut it hot.

Berries sank to the bottom

The batter was too thin, often from soft butter or extra milk. It should be stiff enough to hold a spoon upright for a moment.

Topping melted into a flat layer

The butter in the crumbs was too warm. Chill the crumb topping and pinch it into larger clumps before scattering.

Storage

Cover and keep at room temperature for up to 2 days; the berries make it moist, so refrigerate for up to 5 days after that. Warm slices briefly before serving. Freeze individual squares for up to 2 months.

Safety notes

Wait until the batter is baked before tasting it; it contains raw egg and flour. Contains wheat, egg and milk.