Start with the crust. Combine the flour, sugar, salt, and cocoa powder in a large bowl. Stir together with a fork. With a pastry cutter, or two knives, cut in the butter until the pieces are very small (size of peas) and butter is well incorporated in the mixture.
Separate the egg - place the yolk in a small bowl and the whites in another small bowl. Using a fork, beat the yolk with the heavy cream for about 20 seconds. Stir in the egg + heavy cream mixture with the flour mixture.
Stir and knead until combined. You will think there isn't enough moisture, but just keep kneading and it will come together. (Read the post for more notes on this)
Wrap the dough in plastic wrap and place in the fridge while you work on the filling.
Filling:
Combine the strawberry jam, lemon juice, and almond extract in a very small pot over low-medium heat. Stir. In a small bowl, whisk together the cornstarch and water until the cornstarch is completely dissolved.
Slowly add the cornstarch to the jam mixture and whisk. Let the mixture simmer over low-medium heat for about 7 minutes, stirring occasionally. Remove from heat and let stand while you roll out the dough.
Glaze
Melt the butter and combine it with the powdered sugar. Stir until combined. It will be thick and dough-like. Add in the vanilla and food coloring and stir again. This glaze is supposed to be very thick. It will melt to the perfect consistency on the warm pastries.
Putting it together:
Roll out the dough to 1/8th an inch thickness (very thin!) Using a heart cookie cutter, cut out as many pieces as you can. Then combine the dough and re-roll continuing to cut as many hearts as possible.
I found it easiest to roll out the dough between two sheets of plastic wrap (parchment paper would work too) or make sure to have a well-floured surface*
Spread a layer of nutella on half of the cut-out hearts. Top with about 1/2 tbsp. or a rounded teaspoon of the jam mixture on each heart. Make sure to spread the jam mixture so the heart is filled with it, but making sure to avoid the edges, Place another heart cut out on top, and crimp the edges with the end of a fork (otherwise the mixture will ooze out).
Place the pastries on a tray and cook for about 7-8 minutes (Mine all took exactly 7 minutes)
Remove from the oven and transfer to a wire cooling rack. Let cool for one minute and then spread the glaze over the pastries. The thick glaze with melt and top the pastries perfectly. Top with sprinkles and let cool completely.
Notes
*Tip: mix some flour with cocoa powder to flour your surface so your pastries don't have weird coloring.