Add the sugar, flour, salt, and milk to a small saucepan set over medium heat.
Continuously whisk the mixture together until it starts to bubble up and thickens to a pudding-like consistency.
Transfer the mixture to a heat-safe bowl and cover tightly with plastic wrap. Leave the mixture in the fridge to chill until the cookies are done baking.
Take out the chilled cookie dough and place it on a lightly floured work surface. Roll the dough out until it is ¼” thick. Use a 2” linzer cookie cutter to stamp out the cookies.
Gather the scraps of dough and re-roll to ¼" thick, and cut out more cookies. Repeat 1 more time.
Use a small heart shaped cookie cutter to stamp out a heart in the center of half of the cookies.
Carefully transfer all of the cookies to a parchment paper lined cookie sheet. Place the sheet in the freezer to chill for 10 minutes. In the meantime, preheat the oven to 350°F / 175°C.
Bake the cookies for 8-10 minutes, or until they no longer look wet and have started to slightly turn golden brown on the edges.
While the cookies come to room temperature, finish making the ermine frosting. In a medium sized mixing bowl or in the bowl of a stand mixer fitted with the paddle attachment, beat the butter until it’s light and fluffy.
While the mixer is running, add in the milk mixture one heaping tablespoon at a time, allowing it to fully combine with the butter before adding in more. Repeat until you have used up all of the mixture, then beat in the vanilla.
(Optional) Remove ½ of the buttercream from the bowl. To the buttercream in the mixer, add in the crushed freeze-dried raspberries. Beat together until well combined.
If you are only using vanilla buttercream, transfer the frosting to a piping bag fitted with your tip of choice. If you are using both raspberry and vanilla buttercream - Place the vanilla buttercream and raspberry buttercream into separate piping bags. Snip a hole at the bottom of each bag. Then, place the two bags side-by-side into another piping bag that’s fitted with your piping tip of choice.