Preheat oven to 140°C (convectional: fan-forced). Line light-coloured baking sheet with Silpat / parchment paper.
(Optional) Using a 2.5-cm cookie cutter, dip it into flour, then mark out rounds on the baking sheet. This is to guide the piping of the cookies later on.
Place espresso in a small bowl. Place sugar and salt in another separate small bowl.
6 grams instant espresso, 35 grams caster sugar, 1.2 grams fine salt
Using a microwave or the stove, heat milk until hot but not boiling. Pour about 1/5 of the milk into the espresso and the rest into the sugar-salt mixture. Stir to completely dissolve the espresso and sugar/salt. Let them cool completely.
32 grams milk
Place butter in a mixing bowl. Let it soften until it comes to 22°C. (I microwaved mine at Medium power for 15 seconds.)
100 grams unsalted butter
Use a wire whisk and whisk butter until it is of a smooth toothpaste-like consistency. Add in sugar-salt-milk mixture by tablespoonfuls, whisking well after each addition until it is fully incorporated. Whisk in the coffee.
Break up the lumps, if any, in the ground almond and whisk into the batter.
25 grams ground almond
Sift in cake flour and fold with a spatula just until no specks of flour remain. Do not overmix.
125 grams cake flour
Fit a close star tip in a thick piping bag (or whatever tip you like). Scoop batter into the piping bag. Note: If you are making a bigger batch, it is easier to pipe with smaller quantities of batter, hence you may not want to fill the piping bag too full.
Short Version: Pipe small mounds of cookies onto the baking sheet, about 2.5-cm wide. Space the cookies about 2 fingers apart. Dab the top gently to flatten it.
Detailed Version: Hold the piping bag straight up, perpendicular to the baking sheet. Position the piping bag about 1.5 to 2-cm above the baking sheet and pipe out the cookie batter until it fills the 2.5-cm rounds that you have marked on the baking sheet. (I count "one-thousand-two" then stop piping.) Stop piping and lift the piping bag straight up. Dab the top gently to flatten it. Repeat.
Bake the cookies one sheet at a time. Bake for 18 to 22 minutes in the middle rack, rotating the baking sheet 180° halfway during baking. My cookies took 20 minutes. If your cookie is bigger or smaller, the baking time will have to be adjusted.
Let the cookies cool on the baking sheet for 5 minutes before transferring them onto a wire rack to cool completely. Store in an airtight container.
NOTES
If using canned butter, do take note if it's already salted. The amount of salt should be 1.2% per 100-grams of butter. Adjust the amount accordingly.
If using your own homemade cake flour, sift in the cornstarch first and whisk it in. Cornstarch will not result in the formation of gluten so you can safely whisk it in first. Sift in the flour next then fold it in.