Preheat the oven to 350°F/180°C. Line cookie sheets with baking parchment paper.
Add the butter and sugar to a mixing bowl and cream together by beating with an electric mixer or stand mixer until light and fluffy.
Add the condensed milk and vanilla and beat again.
In a separate bowl, sift the flour, baking powder and salt together. Then beat it into the butter mixture, a little at a time.
If using add-ins like chocolate chips or sprinkles, stir them in now.
Roll teaspoon-sized balls of the condensed milk cookie dough and place them onto the prepared cookie sheets. Press each one down slightly with a fork or the heel of your hand.
Place the cookie sheets in the oven and bake the cookies for 8-10 minutes or until they just begin to turn slightly golden, but don’t overbake them. Allow the cookies to cool on the trays for 5 minutes, then gently transfer them to a wire rack to cool completely.
For the icing, mix the powdered sugar and milk together until smooth. Drizzle over the completely cooled cookies, and add sprinkles.