Line an 8-inch square baking pan with parchment paper, leaving a couple inches of overhang on two sides. That overhang is your handle for lifting the fudge out later.

Set up a double boiler. Fill a small saucepan with about an inch of water and bring it to a gentle simmer, then set a heatproof bowl on top. The bowl should sit above the water, not touch it. That gentle steam heat is what keeps the chocolate from scorching or turning grainy. Make sure the bowl is completely dry before you start.

Add the semi-sweet chocolate and ⅔ cup of the condensed milk to the bowl. Stir with a silicone spatula until the chocolate is almost fully melted, then lift the bowl off the heat and keep stirring until the mixture is smooth, thick, and glossy. It stiffens fast, so work quickly.

Scrape the mixture into the prepared pan and spread it flat with an offset spatula. Refrigerate for 15 to 20 minutes, just until the surface firms up but still feels slightly tacky.

While that chills, wipe the bowl dry and melt the white chocolate with the remaining condensed milk the same way. White chocolate scorches faster than dark, so pull it off the heat when it's almost melted and stir it the rest of the way.

Stir in the orange extract, then add the gel coloring one small dash at a time, stirring between additions, until you reach the shade you want. If the mixture stiffens while you work, set it back over the warm water for a few seconds to loosen it.

Spread the orange layer over the chocolate base right away, before either layer sets. Scatter the sprinkles on top and press them in gently with your fingertips so they stick.

Refrigerate until firm, 1 to 2 hours. Lift the fudge out using the parchment overhang and cut into squares with a sharp knife, wiping the blade between cuts.