Preheat the oven to 350 degrees F.
In a small saucepan, gently warm the milk over low heat. Place the milk in a medium bowl and add the yeast. Set aside for 15 minutes.
Meanwhile, in a medium bowl, combine the ricotta and sugar and mix until well blended. Add milk and yeast mixture, limoncello and flour, and mix to incorporate. Coat a 9-inch bundt cake mold with butter and flour, pour the mixture into the mold and place in a water bath. Bake for 2 hours, until a toothpick inserted in the center comes out clean.