Preheat the oven to 350 degrees and line a cupcake pan with 12 cupcake liners.

In a small bowl whisk together the flour, cocoa powder, baking powder, baking soda, salt and espresso powder. Then set aside.

Cream the butter and sugar together in a large bowl with an electric mixer on high speed until fluffy, about 2-3 minutes. (If using a stand mixer, use a paddle attachment.)

Then add in egg, egg yolk and vanilla and mix on medium-high speed until pale in color and smooth, about 1 minute.

Next mix in the milk and sour cream on low speed until combined. The mixture will look curdled but don't worry, it will pull together once the dry ingredients are added in.

Add the dry ingredients to the wet ingredients and combine on low speed. Scrape the sides and bottom of the bowl as necessary with a rubber spatula.

Divide the batter evenly among the 12 liners, each should be about three-quarters full. Bake the cupcakes for 17-19 minutes.Let the cupcakes cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

While the cupcakes are baking and cooling make the sorting hats. Make the frosting by adding softened, unsalted butter and salt to a medium sized bowl. Mix the butter with an electric mixer until it is pale in color and fluffy, about 5 minutes. (If using a stand mixer, use a whisk attachment.) Then sift in the cocoa powder. Mix until combined. Then sift in the powdered sugar a little at a time until it is all combined. The frosting will be thick at this point. Add in the heavy cream and mix until the frosting is light and fluffy, about 1-2 minutes. Transfer the frosting to a piping bag fit with a medium circular tip like Wilton 2A.

Remove the filling from 6 Oreos. Pipe the icing into the center of each Oreo to resemble the sorting hat. Move your piping hand in a very small circle to build up the frosting. (I recommend practicing on a plate until you get the hang of it.) Once all the hats have been made, pop them in the freezer until they are frozen solid.

Once the hats are frozen, melt the chocolate and coconut oil together. Let the chocolate cool slightly until warm and liquid, but not super hot. Put the frozen hats on a wire rack on top of parchment paper. Spoon the chocolate over the hats until completely covered. While the chocolate is still liquid on the hats, carefully move each hat at the base just a little bit so they're not sitting in a huge pool of chocolate on the wire rack. This will make it easier to remove them from the wire rack later. Let the hats sit until the chocolate is completely set. You can pop them in the fridge to speed this up.

Add the butter to a large bowl and whip on high speed until pale in color and fluffy, about 5 minutes. Then mix in the cold cream cheese. Add the powdered sugar in a little at a time until all is combined. Whip the frosting on high speed until light and fluffy.

Transfer about ½ cup (90 g) of frosting to four different bowls. Dye one bowl red, one gold, one blue and one green. (If you add in a little Dutch cocoa powder it really helps to deepen the colors.) Transfer each colored frosting to a piping bag and cut off a bit of the tip.

Remove the center of each cupcake using a cupcake corer. Fill 3 cupcakes with red frosting, 3 with gold, 3 with blue and 3 with green.

Rewhip if necessary, then transfer the remaining cream cheese frosting to a piping bag fit with a decorative tip. Pipe frosting on top of the cupcakes and sprinkle with sprinkles if you wish.

Once set, place a sorting hat on top of each cupcake, then serve! Bite into one to reveal the hidden house colour and to be sorted!