Blåbärsvåg

Yes, frozen blueberries work well here because they are cooked down into jam anyway. Add them to the pot straight from frozen with the sugar and lemon juice, and expect the simmering to take a little longer than the 5-6 minutes given for fresh berries. Cook until the berries have burst and the mixture looks glossy before you stir in the dissolved cornstarch.

Regular plain yogurt, sour cream, or quark all work in place of the 150 g of Greek yogurt, since its job is to keep the crumb moist and tender. If you use a thinner yogurt, hold back a little of the 50 ml milk so the batter does not get too loose. A plant-based unsweetened yogurt works too, as the recipe already uses oil rather than butter.

That usually means the jam was still warm or too runny when it went into the pan. Let the jam cool until it is thick enough to hold its shape on a spoon, and make sure you stirred in the cornstarch slurry and let it bubble briefly so it actually thickens. Also spread the first half of the batter into an even layer, so the jam has a firm base to sit on.

The blueberry jam can be made a day ahead and kept covered in the fridge, which is convenient because it needs to cool before swirling anyway. Stir it briefly before using so it spreads easily on the batter. The batter itself should be mixed and baked right away, since the baking powder starts working as soon as it meets the yogurt and milk.

Keep the cut squares in an airtight container at room temperature; the oil and yogurt in the batter keep it moist for several days. Dust with powdered sugar only just before serving, because it dissolves into the surface over time. The squares also freeze well once fully cooled - wrap them individually and thaw at room temperature.