Preheat oven to 375° and set a baking sheet on rack in center of oven. Grease a 3/4-cup ramekin with 1/2 tsp. butter, then dust with unsweetened cocoa. Set aside.
Put 1 tbsp. butter and the chocolate chips in a small heatproof bowl set over a small pan of simmering water. Heat, stirring, until chocolate melts; let cool a few minutes.
Whisk egg, sugar, and peanut butter in a small bowl to blend. Whisk in chocolate mixture until well incorporated, then stir in salt and flour just until combined. Pour batter into ramekin. Set on baking sheet and bake until as done as you like, 7 to 10 minutes for a molten center (a 3/4-in. ring around the edge will look dull) or 10 to 12 minutes for a soft center (cake edge will puff slightly).
Let cake cool 2 minutes. Protecting hands, invert onto a plate. Eat right away.