Step 1

Preheat oven to 325 degrees. Fill a medium saucepan half way with water and set to low-medium heat on your stove top. Allow water to come to a simmer or mild boil. Chop chocolate bars into small chunks. Cut butter into small squares.

Step 2

Once water is bubbling, place a medium sized glass bowl over the sauce pan, creating a double broiler. Add chopped chocolate into the bowl. Once chocolate begins to melt, add in butter and stir together. As butter and chocolate melt together, continuously stir together so they become a smooth chocolate mixture. Once everything is melted and smooth, turn off heat and remove bowl from sauce pan. Place chocolate mixture in the refrigerator to cool for 10 minutes.

Step 3

Remove mixture from refrigerator. Pour sugar into the chocolate mixture, using a spoon to stir together. Once sugar is combined with chocolate, add in eggs one at a time, thoroughly stirring in between each egg. Add in chocolate extract and syrup. Continue stirring until combined. Add in flour and salt. Mix everything together. Batter should be thick.

Step 4

Line an 8x8 inch baking dish with parchment paper. Pour half of the batter into the baking dish. Using a spoon, pour 3 horizontal lines of marshmallow lines over the batter, one at each end and one in the middle. Lines should be the length of the batter and about an inch wide. Use a butter knife to create a marble effect. Drag the tip of the knife through the batter, perpendicular to the marshmallow lines. Continue to do this throughout the pan to complete the look.

Step 5

Pour the remaining brownie batter on top of what is already in the baking dish. You may have to use a spoon to smooth out the remaining batter. Once batter is smoothed out make 3 more marshmallow lines and create the marble effect.

Step 6

Bake brownies in the oven for 40-45 minutes. Test with a toothpick to make sure center comes out clean. Allow brownies to cool for at least 30 minutes before transferring them to a cooling rack. Once brownies have completely cooled cut them into squares and serve or store in an airtight container to maintain freshness.