Preheat the oven to 350°F. Line a 9x9-inch square pan with parchment paper, leaving an overhang for easy removal.

In a microwave-safe bowl, combine the 1 1/2 sticks (12 tablespoons) salted butter and 1 cup of the semi-sweet (or milk) chocolate chips. Microwave in 30-second intervals, stirring between intervals, until the mixture is melted and smooth.

Stir the 1 cup granulated sugar into the melted chocolate and butter. Add the 1 tablespoon Kahlúa (optional), 2 teaspoons vanilla extract, and 2 teaspoons instant coffee (optional). Stir until combined.

In a small bowl, whisk the 3 large eggs for about 1 minute, until slightly bubbly on top. Immediately stir the whisked eggs into the chocolate mixture until incorporated.

Add the 1/3 cup all-purpose flour, 1/3 cup unsweetened cocoa powder, 3/4 teaspoon baking powder, and 1/4 teaspoon kosher salt. Stir gently until just combined (do not overmix).

Stir in the remaining 1/2 cup chocolate chips. Spread the batter evenly into the prepared 9x9 pan.

Bake for 28–30 minutes, or until the brownies are just set (a toothpick inserted near the center should come out with a few moist crumbs). Remove from the oven and let the brownies cool to room temperature in the pan.

While the brownies cool (or after they are cooled), prepare the chocolate truffle/ganache. Chop the two (2 x 3.2-ounce) semi-sweet chocolate bars and place them in a microwave-safe bowl with the 1/3 cup heavy cream. Microwave in 20–30 second intervals, stirring between intervals, until the chocolate and cream are melted and the ganache is smooth.

Pour about half of the ganache over the cooled brownies and use an offset spatula or the back of a spoon to spread it evenly to the edges.

Drop the 1/3 cup creamy peanut butter in small spoonfuls over the ganache layer, then use a knife or skewer to gently swirl the peanut butter into the ganache to create a marbled effect.

Place the pan in the freezer for 15 minutes to help the peanut-butter/ganache layer set.

After 15 minutes, if the remaining ganache has firmed, briefly rewarm it (microwave 10–15 seconds and stir) until pourable. Pour the remaining ganache over the set peanut-butter layer and smooth to the edges.

Chop the 1 (3.2-ounce) milk chocolate bar and melt it in a microwave-safe bowl in 15–20 second intervals, stirring until smooth. Drizzle the melted milk chocolate over the top truffle/ganache layer.

Allow the finished brownies to set for at least 1 hour in the refrigerator (or about 2 hours at room temperature) before cutting.

(Optional) To make the Caramel Brûlée pieces: line a baking sheet with parchment. In a medium saucepan, cook the remaining 1 cup granulated sugar over medium heat, stirring occasionally with a heatproof spatula, until the sugar melts and turns a golden caramel color, about 8 minutes. Remove from heat and carefully pour the caramel onto the prepared baking sheet. Spread it in a thin, even layer and let sit until hardened, about 5 minutes. Once hardened, break or crush the caramel into small pieces with the back of a spoon.

If using the caramel pieces, sprinkle them over the set brownies. Use the parchment overhang to lift the brownies from the pan and cut into bars.