Add in the egg and vanilla extract and beat until fully incorporated. Add half of the dry ingredients to the wet ingredients and beat just until incorporated. Then add the remaining dry ingredients and the milk and mix until thoroughly combined.
Allow the cookies to cool on the pan for 15 minutes before transferring them to a wire rack with a spatula to cool completely. Once cooled, melt the chocolate melting wafers in the microwave at 30-second intervals, stirring between each one until fully melted. Transfer to a piping or ziploc bag and cut the corner off and drizzle over the tops of the cookies and then top with sprinkles if desired.
These cookies are best stored in an airtight container at room temperature. They can then be enjoyed for up to 3 to 4 days. Any longer, and I find that the cookie starts to dry out a bit.