Make cookies: In a medium bowl, sift dry ingredients (flour, baking powder, salt) together.
In a large bowl, beat butter, egg, sugar and vanilla on medium speed with an electric mixer. Slowly add dry ingredients and mix until cookie dough forms.
Roll dough into a ball and wrap with plastic wrap. Refrigerate for one hour.
Remove dough from fridge and let sit at room temperature for 5 minutes.
Preheat oven to 325 degrees and line a baking sheet with parchment paper.
Roll cookie dough on a floured surface. Cut into circles with a cookie cutter or an upside-down glass.
Place cookies on lined baking sheet, about an inch apart. Bake for 12-14 minutes and cool on a wire rack.
Prepare buttercream: Beat butter, icing sugar and milk with an electric mixer (starting on the lowest speed, making your way up to the highest). Tint the buttercream with violet gel food coloring and mix until buttercream is completely colored.
Make unicorn horn: Roll white fondant into 1-inch spheres. Roll each sphere into long strips (about 3-4 inches long) then fold slightly. Start wrapping both ends and twist. Brush with silver food dust.
Decorate cookies: With an offset spatula, frost each cookie. Top with sprinkles and one fondant horn.