Dough: In a small saucepan, warm the milk over low heat until it reaches 95-100 degrees.
Pour the 2/3 cup sugar and yeast into the bowl of a stand mixer fitted with a dough hook. Pour the warm milk over the sugar and yeast; stir to combine. Let stand until the yeast is foamy, about 5 minutes.
Add the softened butter, eggs, grated lemon zest, and sea salt. Slowly add the flour a cup at a time and beat at medium speed until a soft dough forms, about 3 minutes. Increase the speed to medium-high and beat until the dough is soft and supple, about 5 minutes longer.
Form the dough into a ball and transfer it to a large lightly buttered bowl. Cover the dough with plastic wrap and let stand in a warm place until doubled in size, about 1 to 2 hours.
Line the bottom of a 9 x 13-inch baking pan with parchment paper, allowing the paper to extend up the long sides. Butter the paper and sides of the pan.
Once the dough has doubled in size, turn the dough out onto a lightly floured work surface. Using a rolling pin, roll the dough into a 10 x 24-inch rectangle.
Filling: In a medium bowl, crush the frozen berries with the back of a big spoon. Stir in 1/4 cup plus 2 tablespoons sugar and cornstarch; toss to coat.
Spread the raspberry filling evenly over the dough. Tightly roll up the dough to form a 24-inch-long log.
Working quickly, cut the log into quarters. Cut each quarter into 4 slices and arrange them in the baking pan, cut sides up. (You will have one extra roll.) Scrape up berries and juice from the work surface and pour over rolls in the pan. Lightly cover the rolls with parchment paper and a tea towel, allow them to rise in a warm place until they are puffy and have filled the baking pan, about 2 hours.
Preheat the oven to 400 degrees. Bake the rolls for about 25 minutes, until they are golden and the berries are bubbling. (Loosely cover the rolls with foil halfway through cooking to prevent them from getting too dark.) Transfer the pan to a rack to cool for 30 minutes. Meanwhile, prepare glaze.
Glaze: In a small bowl, whisk the confectioners' sugar with melted butter and heavy cream until the glaze is thick and spreadable.
Invert the rolls onto the rack and peel off the parchment paper. Invert the rolls onto a platter. Dollop glaze over each roll and spread with an offset spatula. Serve warm or at room temperature.
Mommy's little helper!
Variation: The sweet rolls can be filled with a variety of frozen fruit. Try blackberries, strawberries, blueberries, or chopped sweet cherries.