Make the almond syrup: In a small saucepan, combine the water, sugar, and almond extract. Heat over medium, stirring frequently, until the sugar dissolves. Remove from heat and let cool slightly.
Make the almond filling: In a bowl, beat together the almond butter, butter, and sugar until smooth. Beat in the eggs one at a time, then stir in the almond extract and flour until combined.
Split the croissants in half horizontally. Brush the insides and outsides with the almond syrup.
Spread about 2 tablespoons of the almond filling on the bottom half of each croissant. Replace the tops and spread another 1 tablespoon of filling on top of each croissant.
Top the croissants with the sliced almonds, pressing lightly so they adhere.
Bake at 375°F for 12-15 minutes until golden brown.
Allow to cool slightly, then dust with powdered sugar before serving.