Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius) and grease a donut pan.
In a bowl, whisk together the flour, baking powder and salt. Set aside.
In a separate bowl, whisk together the melted and cooled butter, egg, brown sugar, granulated sugar, vanilla extract, milk, and yogurt until combined.
Add the dry ingredients to the wet ingredients and whisk until a batter forms. Add the mini chocolate chips and fold until evenly incorporated throughout the batter.
Gently spoon the batter into the donut cavities, or pipe them in using a piping bag/ziplock bag with the tip cut off (preferred method). Bake for 11-12 minutes until they are golden brown. To test, gently press your finger into the donut. It is ready if the donut bounces back.
Let the donuts cool in the pan for 5 minutes, then remove them onto a cooling rack to cool completely.