Preheat oven to 350 degrees.Grease a 9x13" baking pan (see notes for baking in different-sized cake pans). Combine milk and lemon juice in a small bowl. Set aside.
Beat butter, sugar, eggs, and vanilla together until smooth.
Add milk and beat until combined.
Add baking soda, salt, baking cocoa, and flour. Beat on low for 30 seconds then on med-high for 3 minutes. Scrape sides with a rubber spatula, as necessary.
Pour batter into the prepared baking pan. Bake for 30-35 minutes, or until a wooden pick inserted into the center comes out clean.
Allow cake to cool completely.
Beat frosting ingredients together. Spread over cake. Sprinkle with sprinkles, if desired.