Preheat the oven to 350F (180C) and line a baking sheet with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, powdered sugar and granulated sugar on medium speed for 2-3 minutes or until fluffy.
Mix in the milk and vanilla until well combined.
Stir in the flour and salt and mix until fully incorporated.
Using a teaspoon make small balls and put them in the baking sheet about 1 inch apart.
Bake for 15 to 20 minutes, or until golden brown on the bottoms and around the edges.
Let them cool completely on a wire rack and store vanilla wafers in an airtight container.
Notes
If you don't have powdered sugar place 1/2 cup granulated sugar in a blender and blend for 30 seconds to 1 minute.