• Preheat oven to 350°F. Grease and line an 8x8 or 9x9 pan.
• In a bowl, beat butter, granulated sugar, and brown sugar until light and fluffy (2–3 minutes).
• Mix in eggs one at a time, then add sour cream, milk, and vanilla. Mix until smooth.
• In a separate bowl, whisk flour, baking powder, baking soda, and salt.
• Add to the wet ingredients and mix until just combined.
• Stir in chocolate chips.
• Pour batter into pan and smooth the top. Bake for 30–35 minutes, until a toothpick comes out clean.
• Let cake cool completely before frosting.

Make the frosting

• Beat butter until smooth.
• Add powdered sugar and cocoa powder.
• Mix in milk, vanilla, and salt until creamy and spreadable.
• Spread frosting over cooled cake.