Preheat the oven to 350°F (175°C). Line a 8x8-inch baking dish with aluminum foil, or parchment paper.
In a medium heatproof bowl, melt the chocolate and butter over simmering water until smooth. (Make sure the bottom of the bowl with chocolate isn’t touching the simmering water.)
Remove the melted chocolate from heat and stir in the sugar. Cool the mixture, so it doesn’t cook the eggs in the next step.
Add the eggs, almond and vanilla extracts and mix until well combined.
Stir in almond and all-purpose flours along with salt. Be careful not to over mix the batter, or the brownies will get dry and dense. Pour the batter into the prepared baking dish.
Using two teaspoons, drop small amounts of dulce de leche all over the brownie batter. Then using a toothpick (or a knife), swirl dulce de leche into the batter. Sprinkle slivered almonds.
Bake for 20 minutes and check for doneness by inserting a toothpick in to the batter somewhere between the center and the edges of the brownies. If it comes out covered with batter, bake for another 2 minutes and check again. If the toothpick comes out slightly moist with a few crumbs, then it’s ready. It should take no longer than 25 minutes.
Cool completely on wire rack before removing from the pan. Cut into 16 squares, before serving. Store in the fridge for up to 5 days.