Preheat the oven to 350 F degrees. Lightly butter a springform pan.
Combine all the dry ingredients in a large mixing bowl. Set aside.
Place a medium-sized saucepan over low to medium heat. Add molasses and sugar. Whisk constantly until sugar has melted. Whisk in butter one tablespoon at a time. Remove from heat.
Add the milk, eggs, and vanilla to a small bowl. Whisk to combine. Pour the mixture into the warm mixture. Whisking constantly.
Slowly pour the warm mixture into the dry ingredients. Whisk until no lumps remain. The batter will be thick. Rest the batter 15 minutes.
Pour the batter into the springform pan and evenly spread. Bake until the center is set and a toothpick comes out clean, about 40 minutes.
Allow the cake to cool completely before removing from the pan. Dust with powdered sugar.
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Notes
The cake can be kept at room temperature for 3 to 5 days. Cover with cake cloche or loosely cover with plastic wrap.
*If you generally find blackstrap molasses too bitter use regular old fashioned molasses. It will make for a sweeter cake.