• Preheat oven to 350°F.

Prepare the Pan:

• Butter the bottom and sides of the 9x13-inch pan, then add parchment paper in both directions with flaps that hang over all four sides of the pan. Butter the top of the parchment paper.

Make the Brownies:

• In the top of a double boiler (or a bowl set on top of a pot of simmering water), melt the butter and chocolate in the top of a double boiler, whisking until smooth. Cool to room temperature.
  16 tablespoons unsalted butter, cut into 1-inch chunks,, 16 ounces bittersweet chocolate
• While the chocolate mixture cools, in a large bowl, whisk together sugar, eggs, and vanilla extract, set aside.
  ¾ cup granulated sugar, 4 eggs, 1 teaspoon of vanilla extract
• In a small bowl, sift together flour and salt, set aside.
  ¼ cup all purpose or gluten free flour, ½ teaspoon salt
• Whisk in a small amount of the chocolate mixture into the egg mixture, then slowly add the rest and whisk until combined. Gently fold in the flour mixture with a spatula.
• Pour batter into the prepared pan. Bake for 20 – 30 minutes. These will seem slightly underbaked, as they are a gooey brownie. Allow to cool to room temperature. Use a knife to cut the pan of brownies down the center so you end up with 2 nearly square shaped pieces. Place pan in the freezer for about 30 to 60 minutes until the brownies are cold and partially frozen.

Color the Ice Cream:

• For 6 color rainbow, add 1 cup of ice cream to each of 6 bowls. Tint the ice cream using a toothpick to remove gel food coloring from the container and applying it to the ice cream then stirring well to mix. Place all bowls of ice cream in freezer until ready to use one by one. Alternatively, color 1 bowl at a time as you're ready to use it.

Add the Ice Cream Layers:

• Once brownies are cold, remove one of the brownie squares and wrap up in plastic wrap and place in refrigerator. Apply the first layer of ice cream to the top of the other brownie square. Freeze for 30 minutes before proceeding to next layer. Repeat for all layers. When adding the last layer of ice cream, immediately place the other brownie piece on top. Press down lightly so it sticks to the ice cream. Freeze for at least 6 hours or overnight.

Finish the Brownie Ice Cream Sandwiches:

• Using a sharp knife, cut in half, then cut each half into 6 rectangles for a total of 12 sandwiches. Individually wrap each sandwich in plastic wrap and freeze until ready to serve. These should keep well in the freezer if stored properly in a resealable freezer bag.