Preheat oven to 325 F and place a rack in the middle. Line two baking sheets with parchment paper. Use a separate sheet of parchment paper to make a stencil for the heart shapes you are going to pipe the cookie dough onto later (about 5 cm / 2” large heart rings spaced 4 cm / 1-1/2” apart).
To make the dough, beat the butter, salt, both sugars in a stand mixer with the paddle attachment at medium speed for 2-3 minutes. Add in the egg whites, vanilla, and beat on medium speed until the mixture cohesive. Add in the flour and rye flour and mix until just well incorporated, scraping down the bowl as needed. Transfer the dough into a piping bag fitted with a 1 cm wide open star piping tip. Place the stencil parchment paper underneath the clean parchment paper on the baking sheet. You should be able to see the heart shapes on the paper below. Slowly pipe cookies onto the parchment lined baking sheet into the shapes. Carefully remove the stencil under the cookies and use it for the next tray.
Bake the cookies one tray at a time for 20-25 minutes in the preheated oven until the edges are golden brown, rotating the tray halfway through. Cool the cookies to room temperature before glazing them.
To make the glaze, heat the ingredients in a double boiler over low heat until it reaches 30-35 degrees Celsius, while stirring the chocolate, and careful not to overheat it. Take it off the heat when the glaze is ready. Dip each cookie half and let the excess drip off. Place the cookies back onto the parchment lined tray. Before the chocolate sets, sprinkle each one with crushed dried rose petals. Let the chocolate set and cool completely before serving.