Preheat oven to 450F (235C) and line a baking sheet with parchment paper. Set aside.

In a large bowl (or in the bowl of a stand mixer) use an electric mixer to beat together sugar and butter until creamy and well-combined.

Add egg and vanilla extract and beat on low-speed until mixture is pale yellow and well-combined.

Stir in sour cream.

In a separate bowl, whisk together flour, cocoa powder, baking soda, baking powder, and salt.

Add about ⅓ of the flour mixture to the wet ingredients and use a spatula to stir the flour mixture in until mostly combined.

Add about half of your very hot/boiling water and gently stir until mostly combined.

Repeat the steps above, alternating adding remaining flour and water until all ingredients are completely combined.

Drop batter by slightly rounded Tablespoon-sized scoop onto prepared baking sheet, spacing cookies at least 2” (5cm) apart.

Transfer to preheated oven and bake for 5 minutes, then allow cookies to cool for 5-10 minutes on baking sheet before carefully removing with a spatula to a cooling rack to cool completely before filling. While cookies are cooling, prepare filling.

White Chocolate Filling

Place white chocolate in a microwave-safe dish and heat in the microwave in 15-20 second increments, stirring well in between, until chocolate is completely melted and smooth. Set aside and allow chocolate to cool until it is no longer warm to the touch before proceeding.

Beat butter in a large bowl using an electric mixer or stand mixer until smooth and creamy. Drizzle in melted, cooled chocolate and beat until completely combined.

Gradually add powdered sugar and stir until completely incorporated, then stir in extracts and salt.

Add heavy cream and slowly increase mixer speed to high, beating for 15-30 seconds on high speed.

Dollop or pipe filling onto the bottom of one cookie and then top with another.

If desired, you can drizzle whoopie pies with melted chocolate and sprinkle with crushed candy canes.