In a small saucepan, combine equal parts water and granulated sugar over medium heat. Stir continuously until the sugar completely dissolves, around 3-5 minutes. Remove from heat and set aside about ⅓ of the syrup for brushing the croissants.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. In a mixing bowl, beat ½ cup of softened unsalted butter until creamy. Gradually blend in 1 cup of almond flour, 1 cup of powdered sugar, 2 tablespoons of matcha powder, and a pinch of salt until smooth. Mix in 2 large eggs one at a time.

Carefully cut your day-old croissants in half lengthwise. Brush the cut sides with the reserved simple syrup. Pipe a layer of the creamy matcha frangipane onto each bottom half, then place the tops back on, finishing with a swirl of remaining frangipane on top.

Sprinkle a handful of sliced almonds over the tops of the filled croissants. Place the prepared croissants on the lined baking sheet and bake for 25-30 minutes, or until golden brown and the edges are crisp.

Once baked, remove the croissants and let them cool on a wire rack for about 10 minutes. Dust the tops lightly with powdered sugar before serving.