• Preheat oven to 325°F. Grease a donut pan very well and set aside.

For the topping

• In a small bowl, whisk melted butter, brown sugar, cinnamon, corn starch, milk and vanilla together, stirring until completely smooth. Set aside.

For the donuts

• In a large bowl, whisk flour, sugar, baking powder, cinnamon, nutmeg and salt together. Make a well in the center and add buttermilk, eggs, vegetable oil and vanilla. Stir until everything is completely combined, being careful not to over-mix. You want to stop mixing when everything is just combined. To make distributing the batter throughout the donut pan wells easier, pour batter into a large resealable bag and snip one corner off.
• Distribute about 1 tbs of the topping throughout each well, about 1-2 tablespoons each (PLEASE NOTE: Do not over-do the cinnamon topping because it may result in it oozing up the sides of the donuts and out of the pan into the bottom of the oven. Do not add more than 2 tablespoons in each well.)
• Pipe donut batter on top of the topping layer, filling ¾ to the top.
• Bake donuts for 10-12 minutes or until lightly browned and the tops spring back slightly when pressed with your finger. Allow donuts to cool for 5 minutes in pan before transferring to a wire rack to finish cooling. When you invert the donuts onto the wire rack, you may find that some of the topping doesn't stick, but the majority of it will. If this happens, just take a knife or spoon and spread it back on top of the donuts.
• If your donut pan has only 6 wells like mine, repeat the above steps and re-grease the pan for the next batch.

For the glaze

• Whisking powdered sugar, vanilla and heavy cream together, adding more or less heavy cream as needed to achieve the consistency you like. I like mine to be fairly thick. Once donuts are completely cooled, go ahead and drizzle the glaze onto each donut.