Chocolate Cupcake With Vanilla Buttercream Frosting

Cupcakes:
2 c all-purpose flour
1 tsp baking soda
1/2 tsp salt
1 bar unsalted butter, softened
1 c granulated sugar
1 c light brown sugar. packed
4 large eggs, at room temperature
6oz sweetened chocolate, melted
1 c buttermilk
1 tsp vanilla extract

Frostings:
1 bar unsalted butter
6 c confectionaire's sugar
1/2 c milk
2 tsp vanilla extract

Preheat oven to 350F
Melt chocolate, place in a double boiler over simmering water on low heat. Stir occasionally until completely smooth. Remove from the heat and let cool.
In a small bowl, sift together the flour salt and baking soda. Set aside.
On medium speed, cream the butter until smooth. Add sugar and beat until fluffy, about 3 minutes. Add eggs, one at a time, beating well after each addition. Add melted chocolate, mixing until well incorporated. Add dry ingredients in three parts, alternating with the buttermilk and vanilla. With each addition, beat until the ingredients are incorporated, but do not overbeat. Using a rubber spatula, scrape down the batter in the bowl to make sure the ingredients are well blended and the batter is smooth. Carefully spoon the batter into the cupcake liners, filling them about 2/3 full. Bake for 20–25 minutes, or until a cake tester inserted in the center of the cupcake comes out clean.
Cool the cupcakes in the tins for 15 minutes. Remove from the tins and cool completely on a wire rack before icing.

Frosting:In a large mixing bowl. Place butter. Add 4 cups of the sugar and then the milk and vanilla.On a medium speed, beat until smooth and creamy, about 3-5 minutes. Gradually add the remaining sugar, 1 cup at a time, beating well after each addition (about 2 minutes), until the icing is thick enough to be of good spreading consistency.

Note:
To make buttermilk, Add 1 tbsp of cream of tartar/lemon juice to 1 c of whole milk. Stir and let stand at room temp for 15 minutes. After 15 minutes, Stir and wait till it curdles.