To the bowl of a stand mixer with the dough hook attachment, add the warm milk, 1 tablespoon of sugar, and yeast. Mix together and let sit for about 5 minutes, until the yeast becomes foamy.
Add eggs, butter, the remaining 4 tablespoons of sugar, all-purpose flour, bread flour, and salt. Mix on low speed until combined. Continue kneading on medium speed for about 5 minutes, until the dough is smooth.
Spray a medium bowl with nonstick cooking spray. Place the dough in the bowl, turning it to grease all sides. Cover the bowl and let the dough rise in a warm place for 1-2 hours, until it has doubled in size.
Line a large baking sheet with parchment paper. Set aside.
Once the dough has doubled, transfer it to a floured work surface and divide it into 2 equal portions.
Roll each portion into a 10-inch circle. Using a pre-cut heart-shaped template (made from parchment paper or cardboard) as a guide, gently shape each circle of dough into a heart by stretching and pinching the edges as needed.
Transfer one of the heart-shaped dough pieces to the lined baking sheet, leaving the other on the work surface for assembly.