Warm the milk (microwave for 30 seconds) and add it to the mixing bowl.

Add 40g of sugar to the warm milk and mix until dissolved.

On the food scale, measure out 100g of sourdough starter and add 2 eggs to the mixing bowl.

Mix these ingredients together until well combined.

In a separate bowl, mix 550g of flour and 10g of salt. Add this dry mixture to the wet ingredients in the stand mixer. The mixture should appear crumbly.

Rest the dough for 10 minutes.

Mix in 173g of room temperature butter until the dough becomes silky (this may take 10-15 minutes). Pause the mixer as needed to scrape the dough from the paddle for full butter incorporation.

Transfer the mixed dough to the bowl used to measure the flour.

Let the dough rest for 12-15 hours.

Melt 113g (1 stick) of butter.

Mix melted butter with ½ cup sugar and 3 tbsp cinnamon.

Preheat the oven to 350 degrees.

Roll the rested dough into a large square and cut it into 16 equal squares.

Place a spoonful of filling on each square, leaving ½ inch of plain, exposed dough on one edge for sealing.

Roll up each square individually.

Cut down the middle of the roll, leaving ½ inch uncut at the end.

Twist one side clockwise and the other counter-clockwise, shaping them into a heart. Twist and connect at the bottom.

Place hearts on a parchment paper-lined baking sheet.

Scramble an egg in a bowl and brush it lightly over the top of the hearts for a golden brown finish.

Bake for 15-18 minutes.

Allow the hearts to cool on a cooling rack.