Make the syrup: In a small saucepan mix the 1 cup water, 2 T sugar, and 1 tsp vanilla extract and pinch of rum extract. Bring to a simmer for 1 minute. Stir util the sugar dissolves then remove from the heat and let cool to room temperature.
Make the Almond Filling: In the bowl of a stand mixer fitted with the whisk attachment mix the ½ cup sugar, almond flour, ¼ tsp kosher salt until combined. Add the butter and mix on medium speed for 1 minute, until well combined. Finally, add the eggs, 1 at a time, mixing on medium/high speed after each addition until the mixture is fluffy and creamy with a frosting-like consistency.
Adjust your baking rack to the center of the oven. Preheat the oven to 350°F. Line a baking sheet with parchment paper.
Slice each croissant horizontally (like you would for a sandwich). Dip just the outsides (the exterior) of the croissant into the syrup. The croissant will be moist, but not soaking wet.
Place the dipped croissants on the baking sheet, cut side up. Spread 2 tablespoons of the almond filling on the bottom half the croissant. Place the top half of the croissants on. Then add 1 tablespoon of almond filling to the top of the croissant. Finish by sprinkling with sliced almonds.
Bake for 15-18 minutes or until golden. Transfer to a wire rack to cool until just warm. Dust with powdered sugar right before serving. Enjoy warm or at room temperature. (These are best enjoyed the same day they are made)!
Notes
Store almond croissants at room temperature in an airtight container for up to 3 days (however, they are best enjoyed day of!). To freeze, let cool completely, then wrap individually and freeze for up to 2 months. Remove and let thaw in the fridge then warm in microwave or oven.