Preheat your oven to 350°F (175°C) and lightly grease your donut pan with non-stick cooking spray.
In a large bowl, whisk together the flour, sugar, baking powder, and salt until well combined.
In a separate medium bowl, beat the egg, milk, melted butter, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir with a spatula until just combined; be careful not to overmix, as this can make the donuts tough.
Transfer the batter into a piping bag or a sturdy plastic bag with the corner snipped off, then pipe the batter evenly into the donut cavities, filling each about three-quarters full.
Bake for 10 to 12 minutes, or until the edges are lightly golden and a toothpick inserted into the center comes out clean.
Allow the donuts to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
While the donuts cool, melt the white chocolate chips with 1 tablespoon of heavy cream in 30-second intervals in the microwave, stirring until smooth.
Dip the top of each cooled donut into the melted white chocolate and place them back on the wire rack.
Melt the dark chocolate chips with the remaining tablespoon of cream, then drizzle the mixture over the white chocolate tops using a fork or a piping bag.
Let the glaze set for at least 15 minutes before serving.