Preheat oven to 350°F (177°C). Line a large baking sheet with parchment paper.
In a stand mixer fitted with the paddle attachment, cream together butter, Redpath® Golden Yellow Sugar, and Redpath® Granulated Sugar until completely blended; 3 minutes.
Add egg and sprinkle in the ground hibiscus flowers, and mix until fully incorporated; about 1 minute.
Add vanilla extract and blend until thoroughly combined.
In a medium bowl, whisk together flour, baking powder, and salt. With the mixer on low speed, add about half of the dry mixture to the bowl until just combined. Add the remaining flour and mix until it forms a dough; do not overmix.
Transfer dough onto a flat work surface and form the dough into a flat disc. Wrap dough in plastic wrap and chill in the fridge for 1 hour. When ready to use, allow dough to come to room temperature; about 5 minutes.
Place dough between 2 sheets of parchment. Roll dough to ⅛ inch thickness. Using a 3-inch heart-shaped cookie cutter, cut out cookies. Gather, reroll, and cut the scraps. Transfer cookies onto the prepared baking sheet. Place cookies into the fridge or freezer to chill before baking; about 10 minutes. Place cookies into the preheated oven for 10 to 12 minutes or until edges are lightly golden in colour.
Cool for 5 minutes on the baking sheet. Transfer to a wire rack to cool completely.
Make the icing as directed in the linked recipe, making only half of the recipe and adding the lemon juice when the recipe calls for vanilla extract. Divide the icing between 6 small bowls or the number of colours being used.
With a toothpick or skewer, place a tiny dab of gel food colouring into each bowl, one colour per bowl. Mix well until the food colouring is thoroughly incorporated. Make sure colours are pale/pastel in colour.
The sixth bowl is for the writing on the hearts, which can be white or, if desired, add 2 to 3 drops of red food colouring to mimic the writing on conversation heart candies.
Fit 5 small piping bags with small round tips (#4 or #5) and place the coloured royal icing into each bag. This will be for outlining and flooding the hearts.
Fit a small piping bag with a very small round tip (#1 or #2) and place the plain/white (or red) royal icing into the bag.
Outline and flood the completely cooled heart cookies with the different coloured royal icing. Set iced cookies onto a wire cooling rack to crust and dry before writing messages; at least 20 minutes.
With the piping bag filled with the white (or red) royal icing, write messages or words onto each cookie. Set aside to dry completely at room temperature; 6 hours to overnight.
Store in an airtight container at room temperature for up to 5 days or in the fridge for up to 10 days. If freezing, line an airtight container with parchment or wax paper and layer completely cooled and if decorated with royal icing, dry cookies with parchment/wax paper.