Preheat your oven to 450°F and place a 10-12 inch cast iron skillet (or similar oven-safe skillet) on the middle rack to heat throughout preheating. This initial heat is crucial—a properly heated skillet creates the characteristic puffy, golden edges that define a great Dutch baby. While the oven preheats, let your eggs come to room temperature if they aren't already, as this helps create a smoother, more cohesive batter.

Combine the melted and cooled butter, milk, flour, room temperature eggs, sugar, vanilla, salt, and lemon zest in a blender. Blend on high speed for about 30 seconds until the batter is completely smooth and homogenous—this creates the silky texture that results in a tender Dutch baby. I like to let the batter rest for a minute after blending; it helps any air bubbles escape and gives the flour a moment to fully hydrate, which promotes even, dramatic rising in the oven.

Carefully remove the preheated skillet from the oven and place it on the stovetop over medium-high heat. Add the 2 tablespoons of Kerrygold butter (or other high-quality unsalted butter) and let it melt and foam, about 1-2 minutes. Add the fresh blueberries and let them toast for just 30-45 seconds, swirling gently—this caramelizes their exteriors and concentrates their flavor. Immediately pour the batter from Step 2 over the hot blueberries, working quickly so the batter can grab the heat and start cooking right away.

Transfer the skillet back to the oven immediately and bake for 15 minutes, or until the Dutch baby is dramatically puffed and the edges are golden brown. Resist the urge to open the oven door—the sudden temperature drop can cause it to deflate. The center should be set but still have a slight jiggle, similar to a custard. When it emerges from the oven, it will be at its peak puffiness before gently settling as it cools.

Remove the Dutch baby from the oven and work quickly—dust the top generously with sifted confectioners sugar while it's still warm so it adheres beautifully. Transfer to a serving plate immediately and drizzle with warm maple syrup. I find that serving it directly in the skillet at the table creates a nice presentation, but transferring to a plate works just as well. Serve immediately while it's still warm and at its most dramatic height.