Line a 9x13-inch baking pan with parchment paper and spray with nonstick spray. Set aside.
Place the white chocolate chips and sweetened condensed milk in a medium saucepan set over medium-low heat. Melt slowly, stirring often, until smooth.
2 cups white chocolate chips, 14 ounces sweetened condensed milk
While the white chocolate is melting, begin making the pralines.
Add the granulated sugar, brown sugar, whipping cream, butter, and baking soda together in a medium saucepan set over medium-high heat.
1½ cups granulated sugar, 1½ cups brown sugar, 1¼ cups heavy whipping cream, 6 tablespoons unsalted butter, ¼ teaspoon baking soda
Stir often until the butter has melted and the pralines begin to boil.
Meanwhile, pour the melted white chocolate into the prepared baking pan and smooth into an even layer. Set aside.
Use a candy thermometer to check the pralines. When the pralines reach 236°F, the candy is ready.
Remove from the heat and stir the pecans, vanilla, and salt into the pralines. Allow to cool for 3-4 minutes, then stir until the pralines start to thicken and lose their glossiness.
2 cups chopped pecans, 2 teaspoons pure vanilla extract, ½ teaspoon kosher salt
Pour the pralines over the white chocolate and gently lift the sides of the white chocolate, folding over the pralines into the middle of the pan. This will create marbling. Continue until it marbles, then press evenly into the pan.
Allow to cool completely (about 4 hours) before cutting and serving.