Let’s break down the process into manageable steps.

Make the Cookie Dough:

1. Cream Butter and Sugar: Combine the softened butter, granulated sugar, and orange zest in the bowl of an electric mixer. Beat on medium speed until smooth.
2. Add Eggs and Vanilla: Add the eggs and vanilla extract. Beat again, scraping down the sides of the bowl halfway through, until well combined.
3. Whisk Dry Ingredients: Sift together the flour, baking powder, and salt into a separate bowl.
4. Combine Wet and Dry: With the mixer on low speed, add the dry ingredients to the butter mixture, alternating with the milk, until the dough comes together. Begin and end with the dry ingredients.
5. Chill Dough: Divide the dough in half, then flatten both halves into disks and place them in separate resealable plastic bags. Refrigerate the dough until firm, about 2 hours. This is important for preventing the cookies from spreading too much during baking and for making them easier to roll out and cut.

Bake the Cookies:

1. Preheat and Prepare: Preheat the oven to 350°F (175°C). Line 2 baking sheets with parchment paper.
2. Roll and Cut Dough: Roll out both pieces of dough until about 1/4 inch thick. Cut out as many cookies as you can with 2- to 3-inch butterfly cookie cutters.
3. Bake: Place the cookies on the prepared baking sheets and bake until they’re just set but not browned, 8 to 10 minutes.
4. Cool: Remove from the oven and let cool 5 minutes on the baking sheets, then remove to racks to cool completely before decorating.

Make and Apply the Icing:

1. Make Icing: Combine the sifted powdered sugar, milk, and egg whites in the bowl of an electric mixer and beat on medium-high speed until thick and glossy. Add more powdered sugar if the icing seems too thin. The consistency should be thick enough to coat the top of the cookie without running off the sides.
2. Divide and Color Icing: Divide the icing among 3 or 4 smaller bowls.
3. Marble Icing: Add 2 to 3 drops each of red and orange food coloring to each bowl, scattering it over the surface of the icing. Drag a toothpick through the drops a few times to make swirls. Don’t overmix, or the colors will become muddy.
4. Dip Cookies: Dip the top side of each cookie in the icing and let the excess drip off.
5. Dry Icing: Place the iced cookies on a rack to dry completely, about 1 hour. When the first bowl of icing becomes too muddled, move on to the next.

These are clear instructions.