Line an 8-inch x 8-inch baking pan with crisscrossed parchment paper and lightly spray with cooking spray.

In a heatproof bowl, combine semisweet chocolate chips, sweetened condensed milk, and chopped bittersweet chocolate. Microwave in 30-second intervals, stirring until fully melted (about 90 seconds total). Transfer to a medium pot.

In a small bowl, mix Marshmallow Fluff, espresso powder, butter, and Kahlua until smooth. Add this mixture to the pot and cook over medium heat for about 1 minute, stirring until heated through. Pour into prepared pan and smooth the top.

Microwave the white chocolate chips with the remaining sweetened condensed milk in same way until smooth (around 90 seconds). Move to the reserved pot.

Repeat mixing the remaining Marshmallow Fluff, butter, Baileys, and Kahlua, and scrape into the pot with the white chocolate mixture. Cook on medium heat, stirring until heated through (about 1 minute). Pour this layer over the semisweet fudge and smooth the surface.

Refrigerate the fudge until fully set (at least 2 hours but up to 5 days).

Lift the fudge out using the parchment overhang, transfer to a cutting board, dust with espresso powder, and cut into squares. Top each piece with an espresso bean if desired.