Start by preheating your oven to 325°F (165°C). Prepare a 12-cup muffin pan by lining each cup with a cupcake liner. This will help your cupcakes bake evenly and give them that cute, classic look.

In your stand mixer, beat the softened butter and granulated sugar together on medium speed for about 3 minutes. You’re aiming for a light, fluffy texture here, this is the foundation of your cupcake batter!

Incorporate the eggs, and vanilla extract into the bowl, then mix until everything is evenly combined. The mixture should be smooth and creamy.

As you go, don’t forget to scrape the bowl’s sides to ensure a smooth, fully combined mixture.

In a separate bowl, sift the cake flour, baking powder, baking soda, and salt together. 

To achieve a light and fluffy texture, gradually add the dry ingredients in portions, switching off with the sour cream and buttermilk as you mix them into the butter base. 

Begin with half of the dry ingredients, mixing on low, then add half of the sour cream and buttermilk.

Repeat with the remaining ingredients. Be careful not to overmix, as you want to keep the batter airy.

Spoon the batter into the liners, filling each about two-thirds full to give them space to rise. A cookie scoop works great for this step to ensure all your cupcakes are the same size and bake evenly.

Pop the muffin pan in the oven and bake for 20 minutes. You’ll know they’re done when the cupcakes spring back when gently touched, or when a toothpick comes out clean. 

Cool the cupcakes in the pan for a few moments, then carefully transfer them to a wire rack to cool entirely.

While the cupcakes are cooling, make the vanilla pastry cream. In a medium saucepan, whisk the egg yolks, sugar, cornstarch, vanilla extract, and a pinch of salt until the mixture turns lighter and slightly thickened. 

Slowly add the milk while whisking constantly to ensure the mixture stays smooth and creamy. 

Cook over low heat, whisking constantly, until the mixture thickens and begins to bubble, taking around 10 minutes. Be patient here! After thickening, strain the mixture through a fine sieve to eliminate any lumps, then stir in the butter to create a velvety finish. 

Seal with plastic wrap and let it cool in the refrigerator.

Now, make the ganache. Chop the chocolate into small chunks and place them in a heatproof bowl.

Warm the heavy cream over low heat until it begins to bubble around the edges, then pour it over the chocolate. 

Give it 5 minutes to melt, then stir until it becomes silky and smooth. Allow the ganache to cool to room temperature, this is when it thickens and becomes perfect for piping.

After the cupcakes have cooled, carefully use a knife to make a small hole in the center of each, avoiding cutting all the way through. You want to leave a little base at the bottom to hold the filling.

Fill a piping bag with your chilled pastry cream and pipe it into the hole of each cupcake, making sure to fill them generously. You can even give the filling a little swirl to make it extra pretty!

Finish by piping the cooled ganache over the top of each cupcake. You can get creative with how you pipe it, smooth, swirled, or even a decorative peak. Enjoy your delicious, homemade Boston Cream Cupcakes!