Mix the egg and heavy cream in a mug, making sure the egg is completely blended.
Stir in the Keto Chow, allulose, cocoa powders, and baking powder.
Microwave on full power for about one minute until it has risen to double the height and is cooked through.
While the cake cools, make the filling. Melt the white chocolate chips with the heavy cream in a microwave safe container. Heat in 10 second bursts, stirring after each, until melted. This should only take about 20 seconds. Stir in the cold butter while the chocolate mixture is still warm.
You can pour the white chocolate over the cake in the mug while it is warm or chill it for a few minutes to firm it up so it is spreadable to make a sandwich.
When the cake is cool, split it horizontally to make two layers and spread the filling in between.
Notes
Use a large mug so the cake has room to rise. No need to oil the mug; the cake should pull away from the sides as it bakes.