Preheat oven to 350°F and line a baking sheet with parchment paper. Set aside.
In a medium mixing bowl, use an electric hand mixer to cream the butter until creamy, about 3 minutes. Add in the eggs one at a time and mix on low speed after each one until combined.
Sift in the almond flour, powdered sugar, matcha powder, and salt. Mix on low speed until combined. Transfer this matcha frangipane to a piping bag and snip about 1” off the tip of the bag.
Slice your croissants in half lengthwise and place the bottom halves on the baking sheet.
Pipe a generous amount of the matcha frangipane on each croissant. Place the top halves of each croissant on top of the frangipane. Use any remaining frangipane to pipe a line on top of each croissant and press sliced almonds on top to stick to the frangipane (optional).
Bake for about 20-25 minutes until deep golden brown and very crispy. Let croissants cool on the baking sheet. Dust with powdered sugar and enjoy while warm or once cooled!