Preheat oven to 400 degrees F. Coat 8 6-ounce ramekins with cooking spray and sprinkle with 2 tablespoons of the sugar, turning to coat the insides. Place them on a large rimmed baking sheet.

In a medium bowl, whisk the egg yolks.

In a medium saucepan, whisk the milk, brown sugar, cocoa, flour, and salt, and cook over medium-high heat, stirring constantly, until the sugar is dissolved and the mixture is slightly thickened and bubbling, 5 to 7 minutes. Remove from heat and stir in the semisweet chocolate until smooth. Slowly whisk the hot chocolate mixture into the egg yolks, whisking constantly. Return the mixture to the saucepan and cook over medium heat until thickened, about 2 minutes. Transfer to a large bowl and let cool to room temperature, stirring occasionally.

In the bowl of a stand mixer fitted with the whisk attachment, beat the egg whites and cream of tartar on high speed until foamy. With the mixer running at medium speed, gradually add the remaining ¼ cup sugar. Increase the speed to high and beat until stiff peaks form. Gently fold half of the egg whites into the cooled chocolate mixture. Fold in the rest of the egg whites. Divide the batter among the 8 ramekins and bake until puffed and just set in the centers, 18 to 20 minutes.

Meanwhile, for the white chocolate sauce, create a double boiler: bring a small saucepan filled with 1 to 2 inches of water to a boil. Reduce the heat to keep it at a simmer. Place the white chocolate chips and the half and half in a heat-proof bowl that will sit on top of the saucepan (but not fall in!). Set it on top of the saucepan and allow the heat of the simmering water to slowly melt the chocolate. Keep an eye on the chocolate and stir it frequently to ensure it doesn’t scorch. Once most of the chocolate is melted, remove the bowl from heat and continue to stir until smooth. Drizzle the white chocolate sauce over the hot souffles, just out of the oven, and serve immediately.