Line an 8x8 baking dish with wet parchment paper to make removing the fudge easier.

In a medium saucepan, combine the unsalted butter, granulated sugar, and half and half over medium-high heat, stirring until melted.

Bring the mixture to the hard ball stage at 237°F, which takes about 10-15 minutes.

In a stand mixer bowl, combine the white chocolate chips, mini marshmallows, and vanilla extract.

Once the sugar mixture reaches the hard ball stage, carefully pour it over the chocolate mixture and blend until smooth.

Pour the fudge mixture into the prepared baking dish, smoothing the top.

Let the fudge cool at room temperature for 15 minutes, then refrigerate for 1-2 hours until fully set.

Remove from the refrigerator, lift out using parchment, and cut into 1-inch squares.