Prepare the Dough : In a large bowl, combine the warm buttermilk and yeast. Let it sit for 5 minutes until frothy. Add the egg, sugar, melted butter, salt, and 2 cups of flour. Mix until well combined. Gradually add the remaining 2 cups of flour and knead the dough for 8-10 minutes until it’s soft and slightly sticky. Cover the dough and let it rise in a warm place for 1-1½ hours, or until it doubles in size.

Make the Raspberry Filling : In a small saucepan, combine the raspberries, water, sugar, and a pinch of salt. Cook over medium heat, stirring, until the raspberries break down. Dissolve the cornstarch in 2 teaspoons of water and stir it into the raspberry mixture. Cook for 1-2 minutes until thickened. Remove from heat and let the filling cool completely.

Make the Lemon Curd : In a saucepan, whisk together the eggs, egg yolk, sugar, lemon juice, lemon zest, and a pinch of salt. Cook the mixture over medium-low heat, stirring constantly, until it thickens enough to coat the back of a spoon (about 5-7 minutes). Remove from heat and stir in the butter until smooth. Transfer to a bowl and let it cool completely.

Assemble the Rolls : Roll the dough out into a 12x18-inch rectangle on a lightly floured surface. Spread the cooled raspberry filling evenly over the dough. Then drizzle the lemon curd over the raspberry filling. Roll the dough tightly from the long side to form a log. Cut the log into 12 equal rolls. Place the rolls in a greased 9x13-inch baking dish, cover, and let them rise for another 30 minutes.

Bake the Rolls : Preheat the oven to 350°F (175°C). Pour the warmed heavy cream over the rolls. Bake for 25-30 minutes, or until the rolls are golden brown and cooked through. Allow them to cool slightly.

Prepare the Frosting : In a bowl, beat together the softened butter, cream cheese, powdered sugar, heavy cream, raspberry jam, and a pinch of salt until smooth and creamy.

Frost the Rolls : Spread the frosting over the slightly cooled rolls. Serve warm and enjoy the burst of zesty lemon and sweet raspberry flavors.