Heat the oven to 220°C / 425°F and line a 12-cup muffin pan with paper liners. The high starting temperature helps build a taller bakery-style dome; position the rack in the center if your oven runs hot.

Mix the flour, granulated sugar, brown sugar, salt, and lemon zest in a medium bowl, then pour in the melted butter and toss with a fork until moist clumps of different sizes form. The crumble should hold together when squeezed but still stay loose and pebbly; add an extra tablespoon of flour if it looks wet and pasty. Chill until needed so it stays chunky during baking.

Whisk together the flour, baking powder, baking soda, and salt in a large bowl, mixing well so the leavening distributes evenly and the muffins rise evenly.

Combine the granulated sugar and lemon zest in another bowl and rub them together with your fingers for 30 to 60 seconds, until the sugar feels damp and fragrant; this releases the lemon oils for a stronger flavor. Add the eggs and whisk until smooth, then whisk in the melted butter and oil, followed by the sour cream, milk, lemon juice, and vanilla, until fully combined. The mixture may look slightly thick and creamy, which is expected.

Pour the wet mixture into the dry ingredients and fold gently with a spatula just until no dry flour remains. The batter should be thick and scoopable, not overmixed; a few small lumps are fine, since overmixing makes the muffins tougher and less domed.

Divide about half the batter among the 12 muffin cups, then add 1 to 1½ teaspoons of lemon curd to the center of each. Cover with the remaining batter, filling each cup about 85 to 90% full. Top each muffin with another small spoonful of lemon curd, about 1 teaspoon, leaving it mostly on the surface rather than stirring it in, then scatter the crumble generously over each one, pressing only very lightly so it adheres. Use a thick lemon curd rather than a runny one, so it stays defined instead of sinking or leaking excessively.

Bake at 220°C / 425°F for 5 minutes, then, without opening the oven, reduce the temperature to 190°C / 375°F and bake for 13 to 17 minutes more. The muffins are done when the tops are high and lightly golden, the crumble is set and golden at the edges, the batter around the center looks baked rather than wet, and a tester inserted into the crumb, avoiding the lemon curd, comes out clean or with a few moist crumbs. Tent the pan loosely with foil for the last few minutes if the crumble is browning too quickly.

Cool the muffins in the pan for 8 to 10 minutes before transferring to a rack, letting them cool until just warm or fully cooled before dusting with powdered sugar. Dust only right before serving, since powdered sugar dissolves over time on warm or moist tops.