Whole-Egg Molasses Buttercream

Bring 1 cup water to just below a simmer in a medium saucepan over medium-low heat. Using a handheld whisk, combine eggs, sugar, and salt in the bowl of a stand mixer. Place bowl on top of saucepan (it should not touch water) and cook egg mixture, whisking constantly, until lightened in color, thickened slightly, and an instant-read thermometer registers 160°F. Remove from heat and beat with stand mixer fitted with the whisk attachment on medium speed until completely cooled and tripled in volume, about 10 minutes. Add butter, 1–2 Tbsp. at a time, beating well after each addition until fully incorporated. Add vanilla and molasses and beat on medium-low speed until just combined.
Buttercream can be made and refrigerated up to 5 days in advance or frozen up to 1 month. To use refrigerated buttercream, let sit at room temperature until soft, at least 2 hours. Beat with stand mixer fitted with the paddle attachment on medium speed until smooth. If texture is not smooth, beat in a little water, 1 tsp. at a time. To defrost, transfer buttercream from freezer to refrigerator to thaw overnight. Then follow the instructions for using refrigerated buttercream.