Preheat your oven to 450°F and set a 10–12 inch cast iron skillet (or similar oven-safe skillet) on the middle rack so it heats up along with the oven. A thoroughly preheated skillet is what creates the characteristic puff and golden edges. Meanwhile, if your eggs are not yet at room temperature, set them out now — they blend into a smoother, more cohesive batter when warm.

Add the melted and cooled butter, milk, flour, room temperature eggs, sugar, vanilla, salt, and lemon zest to a blender. Blend on high speed for about 30 seconds until the batter is completely smooth. Let it rest for a minute after blending so any air bubbles can dissipate and the flour has a moment to fully hydrate, which encourages an even rise.

Carefully remove the hot skillet from the oven and place it on the stovetop over medium-high heat. Add the 2 tablespoons of butter and let it melt and foam for 1–2 minutes. Add the blueberries and let them toast for 30–45 seconds, swirling them gently to caramelize their exteriors and concentrate their flavor.

Immediately pour the prepared batter over the hot blueberries in the skillet, working quickly so the batter starts cooking right away from the residual heat. Transfer the skillet straight back into the oven and bake for 15 minutes, until the Dutch baby is dramatically puffed and the edges are deep golden brown. Do not open the oven door during baking — a sudden temperature drop can cause it to deflate before it has set. The center should be set with just a slight jiggle, similar to a custard.

Remove the skillet from the oven and immediately dust the top generously with sifted confectioners sugar while it is still hot so the sugar adheres. Drizzle with warm maple syrup and serve right away, either straight from the skillet at the table or transferred to a serving plate. The Dutch baby will gently settle as it cools, so serve it while it is still at its most impressive height.