Preheat oven to 425°F. Coat two ramekins with nonstick spray and a dusting of cocoa powder (you really, really want to make sure the cakes are easily removed), place on a baking sheet and set aside.
In a medium heat-proof bowl, combine butter and chocolate and heat for 1 minute and stir well. If the chocolate is not completely melted and combined after stirring, heat again in 10 second increments until it is.
1/4 cup unsalted butter, 2 ounces bittersweet chocolate
Add powdered sugar and eggs and whisk to thoroughly combine.
Add flour, espresso powder and salt and whisk again until just combined and no flour streaks remain.
3 tablespoons all purpose flour, 1 teaspoon espresso powder, Pinch salt
Divide the batter between your two prepared ramekins (it ends up being about 1/2 cup per ramekin). Using a tablespoon or medium cookie scoop, place half of the white chocolate filling in the center of each cake.
Bake for 13 minutes or until the outside edges of the cakes are set, but the center is still a bit jiggy and moist.
Let set for 1 minute, then gently drag a knife around the edges of the cakes (just to be 110% sure they're come out easily) and then cover the top of each ramekin with an inverted plate and use an oven mitt or kitchen towel to carefully flip the ramekins upside down.
If desired, top with powdered sugar, crushed peppermint, chopped peppermint bark, or ice cream (or all of the above if you're living on the edge). Serve immediately and enjoy!