Preheat oven to 300F/150°C and lightly grease the molds. If using regular baking sheets, line with parchment papers or silicone mats.
Dissolve coffee – In a small bowl, stir instant coffee with hot water until the coffee is dissolved. Set aside.
Mix the wet ingredients – With a hand mixer or stand mixer fitted with the whisk attachment, cream together butter and confectioners' sugar until pale white and fluffy for 3-5 minutes. Add egg whites, vanilla extract, and coffee mixture. Beat on low speed until well combined.
Add dry ingredients – Sift in all-purpose flour, cornstarch, milk powder, baking powder, and salt. Beat on low speed until well combined. Don't overmix.
Pipe – Transfer the batter to a piping bag. Cut the tip off about 0.5-inch/1 cm in diameter. Pipe the batter into straight lines on the molds. If using regular baking sheets, pipe into 2-inch/5 cm straight lines, leaving about 2.5-inch/6 cm between each strip because the cookies will spread during baking.
Bake for 13-15 minutes or until the edges are golden brown.
Cool – Remove the cookies from the pan when still warm. Cool completely on a cooling rack.
Store – Immediately store the cookies in an airtight container once they cool down or they will lose their crispness quickly.
Notes
Measuring by weight is recommended for this recipe.