In a small saucepan, combine equal parts granulated sugar and water. Heat over medium heat, stirring gently until the sugar is fully dissolved, about 5 minutes.

In a mixing bowl, beat ½ cup of softened unsalted butter until creamy, about 2 minutes. Gradually mix in 1 cup of almond flour, 1 cup powdered sugar, 2 tablespoons matcha powder, and a pinch of salt.

Preheat your oven to 350°F (175°C) while lining a baking sheet with parchment paper.

Slice day-old croissants in half lengthwise and brush each cut side with the prepared simple syrup.

Pipe a layer of matcha frangipane into the center of each croissant half, then place the tops back on and sprinkle with sliced almonds.

Bake in the preheated oven for 25-30 minutes, or until golden brown.

Cool on a wire rack for a few minutes before dusting with powdered sugar.