Step 1: Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Step 2: In a small saucepan, combine 1/2 cup sugar and 1/2 cup water. Bring to a simmer until sugar dissolves, then stir in vanilla extract. Set aside to cool.
Step 3: In a medium bowl, cream together the softened butter and 1/2 cup sugar until fluffy. Beat in the egg and almond extract, then fold in the almond flour until a smooth paste (frangipane) forms.
Step 4: Sliced the diced apples and toss them with cinnamon. Briefly saut9 them in a pan with a teaspoon of butter for 3-4 minutes until slightly softened.
Step 5: Slice the croissants in half horizontally. Brush the inside of both halves with the prepared syrup. Spread a generous layer of frangipane on the bottom half and top with a layer of the spiced apples.
Step 6: Replace the top half of the croissant. Spread a thin layer of frangipane on top and sprinkle with sliced almonds.
Step 7: Bake for 15-18 minutes until the frangipane is golden brown and set. Let cool slightly, then dust with powdered sugar before serving.