Preheat the oven to 400 degrees Fahrenheit and line two baking sheets with parchment paper.: When you walk into a kitchen warmed to 400 degrees Fahrenheit you will notice an immediate change in the air, a readiness that tells you pastry will crisp quickly. Proper oven heat ensures the puff pastry puffs and sets instead of turning soggy, and parchment prevents sticking while collecting any buttery drips. A common mistake is preheating too late, which can collapse layers, so start this step early. The oven should be fully heated before the danishes go in, and the sound of the initial bake is a light, whispering sizzle as butter renders.
In a medium saucepan, combine 4 tablespoons unsalted butter, 1/2 cup granulated sugar, 1/4 cup water, 2 teaspoons cornstarch, 1/2 teaspoon cinnamon, 1/2 teaspoon nutmeg, and a pinch of salt.: As the mixture warms, the butter melts into a glossy pool and the granulated sugar dissolves, releasing a faint steam scented with cinnamon and nutmeg . The cornstarch will thicken the blend when heated, and the pinch of salt rounds the flavors. Stir continuously to avoid grainy pockets and to let the spices bloom in the fat. A common error is heating too fast, which can cause sugar to scorch; a steady medium flame is key.
Melt over medium heat and bring the mixture to a boil. Boil for 2 minutes, stirring frequently, then add in 2 large peeled and diced Granny Smith apples. Bring the mixture back to a boil over medium-high heat and cook for 5 minutes, stirring often. Remove from heat and let cool.: As the pot reaches a gentle boil the mixture will thicken and become glossy, and you will smell a caramelizing sweetness. Adding the diced Granny Smith apples introduces a crisp fruit aroma and the pieces should soften while holding shape. The brief boil activates the cornstarch so the juices set around the fruit, producing a spoonable, not runny, filling. A typical pitfall is overcooking so the apples turn mushy; watch for them to become tender but still slightly resilient.
Unfold 1 (17.3-ounce) box puff pastry dough (two sheets). Using a rolling pin roll out each sheet to 10×10 inches. Use a pizza cutter and make 12 even strips. Connect two strips together by overlapping by a half inch and pressing together. Twist the strips around itself and press the end to the rest of the dough.: The tactile feel of cold puff pastry sliding under the rolling pin is distinct, cool and slightly resistant. Rolling to a consistent 10×10 inches ensures even baking and predictable portion size. When you twist the strips you create visual interest and layered texture; pressing seams firmly prevents them from separating in the oven. Watch out for over handling, which warms the layers and can reduce the lift, so work briskly and return the pastry to the fridge if it gets too soft.
Place 6 danishes on a parchment lined baking sheet pan then press the middle down. Place a spoonful of the apple filling in the middle of each danish.: Pressing the center creates a little basin to hold the filling, and when you spoon in the glossy apple mixture you will see steam and shimmer that signal the right consistency. The visual cue is a slightly domed edge and a sunken center filled with fruit. Overfilling is a common mistake, leading to spillover and soggy edges, so use measured spoonfuls and leave room for the crumb topping.
Mix 2 tablespoons brown sugar, 1/4 teaspoon cinnamon, and 1 1/2 tablespoons unsalted butter melted in a small bowl. Stir in the 1/4 cup all-purpose flour until the mixture is thick and crumbly.: The crumb topping should feel sandy and slightly moist from the melted butter , with the brown sugar lending a molasses scent. Work the mixture just until coarse crumbs form, which will bake into crisp, slightly chewy bits. If you overwork it into a paste the topping will lose its desirable crumbly texture, so aim for a pebbly consistency.
Mix 1 large egg and 1 tablespoon water together in a small dish then brush each pastry with the egg wash.: The egg wash gives the pastry a glossy, golden surface and helps the crumb stay in place. Brushing should be even but light, you want a sheen not puddles. The sound as you brush is barely audible, but visually you will notice the dough take on a satin sheen. A common oversight is applying too much egg wash which can pool and create dark spots, so tap off excess from the brush.
Sprinkle the crumb topping over the tops of each danish and bake for 16-18 minutes until edges are golden. Let the danishes cool for 5 minutes before glazing.: As they bake you will smell butter and sugar caramelizing, and the edges will rise into crisp, golden ribbons. Look for a deep golden hue on the pastry edges and a set, slightly browned crumb. Letting them rest five minutes helps the filling settle so the glaze adheres rather than slides off. A mistake is pulling them out too early which can leave the center underbaked and the pastry too soft.
In a small bowl combine 1/2 cup powdered sugar, 2 tablespoons heavy whipping cream or milk, and 1/4 teaspoon vanilla extract.: This glaze comes together into a smooth, glossy drizzle, and the powdered sugar dissolves into the liquid, releasing a sweet vanilla perfume. Choose heavy whipping cream for a richer, silkier glaze, or milk for a lighter finish. If the glaze is too thick add drops of the liquid until it flows slowly off a spoon; if too thin add a touch more powdered sugar. Avoid adding too much liquid at once which can make it runny.
Stir the glaze ingredients together and drizzle over each warm apple danish.: The final drizzle adds a cool, creamy counterpoint to the warm pastry and apple filling, and visually it creates appetizing ribbons of white against golden pastry. When you drizzle, the glaze will set into a delicate sheen as it cools, and the aroma of vanilla extract will lift the whole tray. A trap to avoid is glazing while the pastries are too hot, which can cause the glaze to sink into the filling rather than resting prettily on top.