In a large bowl or the bowl of a stand mixer, combine the flour, yeast and salt.
In a small microwave safe bowl (or glass measuring cup), combine the sugar, butter, water and milk. Heat in the microwave until the mixture is hot and the butter is almost melted (it took about 1 minute 30 seconds on HIGH in my microwave). Gradually add the milk mixture into the flour mixture. Add the egg and poppy seeds. Mix until a soft and sticky dough forms.
If using a stand mixer, switch to the dough hook and set on the lowest mixing setting for 4 minutes or until the dough becomes elastic or knead by hand on a lightly floured surface for 3-4 minutes. Cover the dough with plastic wrap and allow it to rest on the counter for 10 minutes. Preheat the oven to 200 degrees.
Once the dough has rested, roll it out on a lightly floured surface until it measures a 16x9 inch rectangle. Prepare the filling by combining all of the ingredients in a small mixing bowl except for the butter. Spread the butter over the top of the rolled dough, then cover with the filling. Tightly roll the dough and cut into 12 portions. I like to use dental floss for this... I also cut it in half, then fourths etc. That way my pieces are all uniform in size.
Place the rolls into a lightly greased 9x13 baking dish and gently press a piece of aluminum foil over top. Turn OFF your oven and place the covered rolls inside to rise. My rolls took 45 minutes to double in size, but they could take up to 1 hour.
Remove the rolls from the oven and turn ON your oven to 350 degrees. Once heated, bake the rolls for 25 minutes or until the centers are lightly golden brown in color. To prevent over-browning, use that same aluminum foil to set over the rolls half way through baking.
Remove from the oven and allow to cool for 15 minutes before frosting. While rolls are cooling, prepare the frosting by combining all of the ingredients in a small mixing bowl and beating by hand or with a hand mixer until smooth. Spread over the tops of the rolls and garnish with additional lemon zest, if desired.
Notes
-Fresh fruit such as blueberries, strawberries or raspberries can be used in the filling.