Marbled Vanilla Cupcakes

Ingredients

• 1 1/2 cups cake flour, sifted
• 1 1/2 tsp baking powder
• 1/4 tsp salt
• 1/2 cup unsalted butter, room temperature
• 3/4 cup granulated sugar
• 2 large eggs, room temperature
• 1 tsp vanilla
• 1/2 cup + 1 tsp whole milk, room temperature, divided
• 1 tsp cocoa powder