Make the Brioche Dough:
In the bowl of a stand mixer fitted with the paddle attachment, combine flour, yeast, and sugar. Mix briefly to combine.
Melt the butter in a small saucepan and let it cool slightly. Whisk in warm milk (100–110°F), egg, vanilla extract, and salt.
Drizzle the wet ingredients into the dry ingredients and mix on low speed until a shaggy dough forms.
Switch to the dough hook, increase speed to medium, and knead for 6–8 minutes, until smooth and elastic. Add 1–2 tablespoons more flour if the dough is very sticky.
Transfer the dough to a lightly greased bowl and cover with plastic wrap or a shower cap.
Let rise in a warm spot for 1 to 1½ hours, or until doubled in size.
Prepare the Strawberry Filling:
While the dough rises, toss diced strawberries with cornstarch in a small bowl. Fold in strawberry preserves.
Transfer the mixture to a small saucepan and cook over medium heat, stirring constantly, until it bubbles and thickens slightly (about 2–3 minutes).
Return the mixture to the bowl and let cool completely.
Assemble the Rolls:
Once risen, punch down the dough and turn it out onto a lightly floured surface.
Roll into a rectangle (about 9x14 inches) and spread the cooled strawberry filling evenly over the surface.
Use a sharp knife or pizza cutter to slice into 8 long strips. Roll each strip into a spiral.
Place spirals in a parchment-lined cast iron pan or 9-inch cake pan, leaving space between each roll.
Cover with plastic wrap or a shower cap and refrigerate overnight.
Bake in the Morning:
Remove rolls from the fridge and let sit at room temperature for 30–45 minutes while the oven preheats to 350°F.
Once puffed, bake for 25–28 minutes, or until golden and fully baked (internal temp with an instant thermometer should read 190–200°F
Make the Frosting:
In a medium bowl, combine powdered sugar, strawberry jam, optional strawberry powder, and 2 tablespoons of milk.
Use a hand mixer (or a whisk and strong arms!) to beat until light and fluffy. Add more milk as needed for your desired consistency.
Spread or drizzle over warm rolls, letting some drip into the spirals. Add more frosting if desired and serve warm.