In a large bowl, sift the flour, salt , cocoa powder and baking soda, then mix them well with a spoon. Set aside.
In the bowl of a stand mixer, whisk the butter, oil and sugar until creamy.
Add the egg and vanilla and whisk more.
In a big cup, mix with a spoon the buttermilk, vinegar and red food coloring (use enough to obtain a bright red colored liquid).
Pour both flour and buttermilk mixture into the bowl. Make sure to strain the buttermilk mixture. Mix well until smooth.
Pour the batter in a 9 inch greased round pan.
Bake in a 350 °F preheated oven for about 25 minutes.
Let cool completely.
Prepare the cream cheese frosting by mixing all the ingredients until smooth.
Cut the cake into half. Trim the dome on top away to make the top layer surface even.
Crumble the trimmed dome to be used for garnish.
Put half the cream cheese on 1 half of the cake and spread it evenly.
Top the frosting with the other half of the cake.
Spread the remaining cream cheese frosting on top and sides of the cake.
Garnish with the cake crumbs.