Pour warm water into a medium bowl. Add 1 teaspoon of sugar and the yeast, whisk to dissolve, and let sit for 8-10 minutes until foamy.

Sift the flour into a large bowl, make a well in the middle, and add sugar, salt, warm milk, eggs, softened butter, vanilla extract, lemon zest, and the yeast mixture.

Whisk the ingredients in the middle of the well. Slowly incorporate the flour into the wet mixture, mixing until a shaggy, sticky dough forms. Cover with a kitchen towel and let rise in a warm place for 45-55 minutes until doubled in size.

Once risen, deflate the dough by lifting and folding it. Transfer to a floured surface and roll into a large rectangle.

In a separate bowl, whisk together the lemon curd and cornstarch. Spread this mixture on top of the dough.

Starting with the longer edge, gently roll the dough into a tight roll. Cut into 12 equal pieces using unflavored floss or string.

Place each roll into a buttered 8" by 12" baking pan and let rise for 15 minutes. Preheat the oven to 355?F (180?C).

Pour heavy cream over the rolls and bake for 25-30 minutes until golden brown. Remove from oven and let cool.

For the frosting, beat softened butter in a large bowl until light and fluffy.

Add powdered sugar and salt, and beat until fluffy. Scrape down the bowl and beat for about 5 minutes.

Add sour cream, vanilla extract, and lemon juice, and beat until very light and fluffy.

Spread the frosting onto the warm rolls and add dollops of lemon curd. Swirl together with a spatula.