Preheat the oven to 350°F (175°C).
In a large mixing bowl, use a mixer to beat the eggs and salt together until frothy.
Add the sugar and continue to beat for 2 to 3 minutes. Add the vanilla extract and beat for an additional 30 seconds.
In a microwave-safe bowl, combine the butter and unsweetened chocolate. Microwave on high for 1 minute. Stir well, then microwave for another 15 seconds if needed, repeating as necessary until fully melted.
With the mixer running, slowly pour the melted chocolate and butter mixture into the egg and sugar mixture. Add the flour and beat for 1 minute. Scrape down the sides of the bowl and beat for an additional 30 seconds.
Line an 8×8 baking pan with foil or parchment paper and lightly spray with nonstick cooking spray. Pour the batter into the prepared pan.
Place the pan on the top rack of the oven and bake for 15 to 20 minutes, or until the center is barely set. The brownies should be slightly undercooked. Allow the brownies to cool completely.
Once cooled, cut the brownies in half, then cut each half into approximately 1/2-inch wide strips. Wrap the brownie strips in foil and chill in the refrigerator for at least 30 minutes.
Melt the semi-sweet or dark chocolate according to the package instructions. Dip each brownie fry into the melted chocolate and place on a baking sheet lined with a silicone mat or parchment paper. Immediately sprinkle with sea salt.
Repeat with the remaining brownie fries, then place the baking sheet in the fridge to set. Serve the brownie truffle fries cold.