Take ½ cup (14 grams) of the fresh mint leaves and, using a sharp knife, finely slice the mint leaves and transfer into a small, heavy-bottomed saucepan. Add in ½ cup (113 grams) of the cubed butter and place over medium heat. Stir with a heatproof spatula to help melt the butter and ensure mint does not burn.
Once butter is completely melted, reduce heat to medium-low. Gently heat the mint in the butter for about 2 minutes or until fragrant. Remove from heat, cover, and allow the mint to infuse into the melted butter; 30 minutes.
Strain the mint-butter mixture through a fine-mesh sieve into a bowl, scraping and pressing down on the mint leaves with the spatula to remove as much as the flavoured butter and liquid from the leaves as possible. Discard solids.
Add the remaining cubed butter into the bowl. With the spatula, cream the cubed butter into the mint mixture until thoroughly combined. This helps the mint-butter to cool down and to reach the consistency of room temperature butter. Place into the fridge for 5 minutes to solidify the butter slightly if it is still liquidy. Stir at the halfway mark to ensure it doesn’t get too hard or cold (see Chef’s Tips).
Preheat oven to 375°F (190°C). Line baking sheets with parchment paper.
In a small food processor (or spice grinder) fitted with a metal blade, combine the Redpath® Granulated Sugar with the remaining ¼ cup (7 grams) mint leaves that have been torn or roughly sliced into small pieces. Process until mint is finely chopped (see Chef’s Tips).
Place the solidified mint butter into the bowl of a stand mixer fitted with the paddle attachment. Add in the Redpath® Golden Yellow Sugar and the sugar-mint mixture. Mix at medium speed for 1 to 2 minutes until completely blended; do not aerate.
Add the egg, vanilla extract, mint extract (if using), and salt. Mix until fully incorporated into the butter mixture; about 1 minute.
In a separate bowl, whisk together flour and baking powder. On low speed, add about half of the dry mixture to the bowl until just combined. Add the remaining flour. Mix until it just 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 the dough to come to room temperature; about 5 minutes.
Place dough between 2 sheets of parchment. Roll dough to ¼- to ⅛-inch thickness.
Using a shamrock (clover) shaped cookie cutter, cut out cookies. Scraps can be gathered and rerolled, and cut. Transfer cookies onto the prepared baking sheets. Place cookies into the fridge or freezer to chill before baking; 10 minutes in the freezer or 20 minutes in the fridge.
Place cookies into the preheated oven for 8 to 10 minutes or until lightly golden in colour.
Cool for 5 minutes on the baking sheet. Transfer to a wire rack to cool completely.