1. Start with the Dulce de Leche Glaze:

• Combine the milks, cream, cornstarch, and cinnamon in a 1-qt. nonstick pan.
• Bring the mixture to a boil.
• Reduce heat to low and continue to cook, stirring occasionally, until the mixture becomes deep caramel in color and thickens (about 2 hours).
• Strain the mixture into a bowl to ensure a smooth texture and set aside to cool.

2. Prepare the Sponge Cake Batter:

• Preheat your oven to 350 degrees F.
• Generously grease each slot of a 12-count doughnut pan with butter. Dust with flour, shaking off any excess.
• In the bowl of a stand mixer, cream together the butter and sugar until smooth and creamy.
• Add the eggs, one at a time, ensuring each is fully incorporated before adding the next.
• Mix in the salt, vanilla extract, and baking powder.
• Gradually add the flour, mixing just until combined.
• Using a spoon or a piping bag fitted with a large round tip, fill each donut mold up to the indicated line.
• Bake for 12-15 minutes, or until the donuts are golden brown on top.
• Remove from the oven and allow the donuts to cool slightly in the pan.
• Gently loosen the donuts using the edge of a thin spoon and transfer them to one or two large baking dishes or high-sided pans (e.g., two 13×9 inch pans).
• Gently poke holes all over the donuts using a toothpick or fork.

3. Prepare the Tres Leche Combo:

• Whisk together the three milks and vanilla extract in a medium bowl until well combined.
• Gently pour this mixture over the donuts, allowing them to absorb as much liquid as possible.
• Spoon any excess liquid that runs off back over the donuts to ensure they’re well-soaked.

4. Glaze and Serve:

• Once the Dulce de Leche glaze has cooled, dip each Tres Leche-soaked donut into the glaze, ensuring it’s well-coated.
• Place the glazed donuts on a tray or wire rack to dry.
• Serve once the glaze has set.