Combine 1/2 cup heavy cream, 1/2 cup whole milk, 1/4 cup granulated sugar and 1/4 cup unsalted butter in a heat-proof bowl or measuring cup. Heat in the microwave in 20-second intervals, stirring between intervals, until the mixture reaches 130°F. Set aside to cool slightly.

In the bowl of a stand mixer, whisk together 3 1/4 cups all-purpose flour, 2 packets rapid rise yeast and 1 teaspoon salt.

Pour the warm milk/cream/butter mixture into the dry ingredients and mix briefly with the dough hook until just combined. Add the 2 egg yolks and mix on low until incorporated.

Knead the dough on low in the mixer with the dough hook for 5 minutes. Remove the dough, transfer to a lightly floured surface and knead by hand for 5–8 minutes, until the dough is smooth and springs back when you poke it.

Form the dough into a ball, place it in a large greased bowl, cover, and let rest for 20 minutes.

While the dough rests, make the pastry cream: Whisk together 1/4 cup granulated sugar, 1 1/2 tablespoons cornstarch and 1 pinch salt in a medium heavy-bottomed saucepan. Whisk in 1 1/4 cups whole milk and 1 large egg yolk until smooth.

Heat the pastry cream mixture over medium heat, whisking constantly, until it thickens and begins to bubble (about 5–8 minutes). Once it bubbles, continue whisking and cook 1–2 more minutes.

Remove the saucepan from the heat and whisk in 1 tablespoon unsalted butter and 1 teaspoon vanilla extract. Transfer the pastry cream to a heat-proof container, press plastic wrap directly onto the surface to prevent a skin, and chill in the refrigerator for at least 4 hours or overnight.

After the dough has rested and the pastry cream is chilling, punch down the dough. Roll it out on a lightly floured surface to 1/2-inch thickness. Use a 3-inch round cutter to cut donuts and place them on a parchment-lined baking sheet. Cover with a clean kitchen towel and let rise about 1 hour, until nearly doubled in size.

Pour vegetable oil into a large, heavy-bottomed pot to a depth of several inches. Heat the oil and monitor with a candy thermometer. Maintain the oil temperature between 350–375°F while frying (about 360°F is ideal).

Fry the donuts, a few at a time without overcrowding, for 2–3 minutes per side until golden brown. Adjust heat to keep the oil in the 350–375°F range.

Remove donuts with a slotted spoon or spider and transfer to a cooling rack set over paper towels to drain. Let the donuts cool until warm or room temperature before filling.

Make the chocolate ganache: heat 1/3 cup heavy whipping cream in the microwave until just simmering (about 45 seconds), then pour over 4 ounces semi-sweet chocolate chips in a heat-proof bowl. Let sit 2–3 minutes, then stir until smooth.

Fill the cooled donuts with the chilled pastry cream: transfer the pastry cream to a piping bag fitted with a small round tip (or use a pastry syringe). Insert the tip into the side or bottom of each donut and pipe in pastry cream until filled.

Dip the tops of the filled donuts into the chocolate ganache, allow excess to drip off, then set the donuts on a rack until the chocolate sets.