In a medium bowl, combine the vanilla cake mix, 1/2 cup butter at room temperature, 4 egg whites, and 1/4 cup sour cream. Use a hand mixer, stand mixer, or simply mix by hand to combine the ingredients just until blended. It's normal if you see small bits of butter in the dough.

Divide the prepared dough in half. To one half, add 2 tablespoons of unsweetened cocoa powder. Mix well to ensure the cocoa is completely incorporated, creating your chocolate dough.

Using a medium cookie scoop, portion out 12 vanilla and 12 chocolate dough balls onto a parchment-lined baking sheet. Refrigerate the dough balls for at least 1 hour to help them firm up. I find that chilling the dough really helps the cookies maintain their shape during baking.

Preheat your oven to 350°F (175°C). After the dough has chilled, roll each mound in your hands until smooth. Arrange 8–10 dough balls at a time on a silicone mat or parchment-lined baking sheet, spacing them apart. Bake for 9–11 minutes, or until the cookies are set. Remove from the oven and let them cool on the sheet for 5 minutes before transferring to a wire rack. Repeat with remaining dough.

Once the cookies are fully cooled, make the strawberry buttercream. In a large bowl, combine 1/2 cup softened butter, 2 1/4 cups powdered sugar, and 1/3 cup strawberry jam. Mix using a hand mixer until the frosting is creamy and smooth. For an extra fruit-forward flavor, I sometimes add a touch more jam at the end.

Transfer the strawberry buttercream to a piping or zip-top bag. Pipe a generous dollop of buttercream onto the flat side (bottom) of half the cookies. Top each with another cookie of the opposite flavor (vanilla with chocolate and vice versa), pressing gently to sandwich them together. Serve immediately or store in an airtight container in the refrigerator for up to 4–5 days. The cookies also freeze well for up to 3 months.