Combine the sugar and water in a small saucepan over medium heat, stirring occasionally until the sugar fully dissolves, about 3-4 minutes. Remove from heat and let the syrup cool completely to room temperature—this prevents it from soaking the croissants too aggressively. While the syrup cools, preheat your oven to 350°F and line a baking sheet with parchment paper. Sift the almond flour into a small bowl to remove any lumps, which ensures a smooth filling texture. I find that having everything prepped and ready makes the assembly process much faster and less stressful.

Using an electric mixer, beat the softened unsalted butter for about 3 minutes until it becomes pale, fluffy, and increases in volume—this incorporates air and creates a lighter texture. Add the sifted almond flour from Step 1, powdered sugar, matcha powder, and salt to the butter. Mix on medium speed for about 1-2 minutes until everything is well combined and the matcha is evenly distributed throughout, giving the filling a beautiful uniform green color. The mixture should be thick and spreadable. I like to stop and scrape down the bowl halfway through to ensure no streaks of matcha remain.

Add the room-temperature eggs one at a time to the matcha-butter mixture, beating well after each addition to fully incorporate before adding the next egg. Add the vanilla extract with the last egg. Continue beating for another minute until the filling is smooth, creamy, and homogeneous. The filling should have a mousse-like consistency that's thick enough to pipe but spreadable enough to create layers.

Using stale or day-old croissants (which have a firmer crumb and won't fall apart when soaked), carefully cut each croissant in half horizontally using a serrated knife with a gentle sawing motion. Arrange the bottom halves cut-side up on your prepared baking sheet, spacing them about 2 inches apart. Transfer the cooled matcha simple syrup from Step 1 to a pastry brush or small spoon, then lightly brush the cut side of each bottom half—use restraint here, as too much syrup will make them soggy rather than moist.

Transfer the matcha filling from Step 3 into a piping bag fitted with a medium round or star tip for better control and presentation. Pipe a generous amount of filling onto each syrup-brushed croissant bottom half, creating a small mound. Gently place the top half of each croissant on top of the filling, pressing down very lightly so it nestles onto the filling without compressing it. Brush the top surface of each croissant with a thin layer of the remaining matcha syrup, then pipe a decorative swirl or dollop of filling onto the top of each one. Finish by sprinkling sliced almonds over the tops for texture and visual appeal.

Place the assembled croissants in the preheated 350°F oven and bake for 25-30 minutes, until the filling is set and the croissants are lightly golden on top. Remove from the oven and let cool on the baking sheet for 10 minutes—this allows the filling to firm up slightly and makes them easier to handle. Transfer to a serving plate and dust generously with powdered sugar using a fine-mesh sieve for an even, elegant finish. Serve while still slightly warm for the best flavor and texture.