Step 1: Prepare Your Setup
Preheat your oven to 350°F (175°C). Line two or three baking sheets with parchment paper or silicone baking mats. Fit a large piping bag with a large open star tip (Wilton 1M or 2D). Set aside.

Step 2: Make the Cookie Dough
In a large mixing bowl, beat the softened butter with an electric mixer on medium speed for about 2 minutes until it’s pale and creamy. Add the sifted powdered sugar and continue beating for another 2-3 minutes until the mixture is light and fluffy. Scrape down the sides of the bowl with a rubber spatula.

Step 3: Add Vanilla
Beat in the vanilla extract and pinch of salt until fully incorporated.

Step 4: Incorporate Dry Ingredients
Sift together the all-purpose flour and cornstarch. Add them to the butter mixture in two additions, mixing on low speed just until combined after each addition. Don’t overmix—stop as soon as no flour streaks remain. The dough should be soft, smooth, and pipeable but hold its shape.

Step 5: Pipe the Cookies
Transfer the dough to your prepared piping bag. Hold the bag vertically about ½ inch above the baking sheet. Pipe 2-inch rosettes by applying steady pressure while moving in a circular motion from the outside in, creating a spiral pattern. Leave about 1½ inches between cookies as they’ll spread slightly. You should get 32-36 cookies total.

Step 6: Bake
Bake for 12-15 minutes, just until the edges are barely starting to turn golden. The cookies should still be pale in the center—they’ll look slightly underdone, but that’s perfect. They’ll firm up as they cool. Don’t overbake or they’ll lose that delicate, melt-in-your-mouth texture.

Step 7: Cool Completely
Let the cookies cool on the baking sheet for 5 minutes, then carefully transfer them to a cooling rack using a thin spatula. Allow them to cool completely before filling—this is crucial or the buttercream will melt.

Step 8: Make the Buttercream
While cookies cool, beat the softened butter in a clean bowl for 2 minutes until creamy. Gradually add the sifted powdered sugar, one-third at a time, beating well after each addition. Add the milk and vanilla extract, then beat on high speed for 3-4 minutes until the buttercream is light, fluffy, and smooth.

Step 9: Assemble the Sandwiches
Turn half of the cooled cookies upside down (flat side up). Spread or pipe about 1 teaspoon of buttercream on each cookie. Add about ½ teaspoon of jam in the center of the buttercream. Top with another cookie, pressing down very gently to sandwich them together. The filling should just peek out from the sides.

Step 10: Serve and Store
Arrange your beautiful Viennese Whirls on a serving platter. They’re ready to enjoy immediately or can be stored in an airtight container.