Using an electric mixer, cream together butter and ricotta. Add vanilla; mix well. Gradually add the sugar, mixing well to combine. Add eggs, one at a time, mixing well after each.
In a separate bowl, whisk together flour, baking powder and salt. Gradually add the dry ingredients to the wet ingredients; mix until combined. Cover and chill in the refrigerator for at least 2 hours, or overnight.
Preheat oven to 350 degrees F. Line baking sheets with parchment or silicone mats.
Drop dough in rounded tablespoons onto prepared baking sheets. You want the dough really cold when it goes into the oven so that the cookies don’t spread, so if the dough seems a bit soft, put the baking sheets back in the refrigerator (or even in the freezer) for about 10 minutes before baking. Bake for 12-14 minutes, or until the bottom of the cookies are golden brown (the tops will still be pale). Cool on the baking sheets for about 5 minutes, then transfer to wire racks to cool completely.
In a medium bowl, whisk together all of the glaze ingredients until smooth. Use 4 tablespoons of milk to start, and add more to thin if necessary. The glaze should be very thick. Dip the tops of the cookies in the glaze, then set on a wire rack. Immediately add sprinkles. Let the cookies rest at room temperature until the glaze sets.
Make AheadPrepare the dough in advance and keep it covered in the refrigerator for up to 24 hours before scooping and baking. You can also bake the cookies in advance and freeze them before icing. Wrapped tightly, the baked cookies will keep in the freezer for up to 2 months. Thaw on the counter and glaze when ready to enjoy!You can freeze the cookies after they're glazed, but the thawed glaze and sprinkles don't tend to look as good.StorageStore the glazed cookies in an airtight container at room temperature for 3-4 days. You can also refrigerate the cookies for up to 1 week.