Preheat the oven to 350 degrees F. Line an 8-inch square baking pan with aluminum foil and grease with butter.
In a medium saucepan, bring 1 inch of water to a low simmer. Place the butter and chocolate in a heatproof bowl over the saucepan without the water touching the bottom of the bowl. Stir frequently until the mixture is fully melted. (Alternatively, melt the butter and chocolate in the microwave in 30-second blasts stirring to distribute the heat.)
Remove the bowl from the heat and stir in the cocoa powder, sugar and vanilla extract. Whisk in the eggs one at a time until fully incorporated. Fold in the flour and almonds. Transfer the batter to the prepared baking pan and dollop the marshmallow creme on top. Drag a knife through the batter to create a swirl pattern.
Bake for 30-35 minutes until the bars are set but still fudgy. Cool completely on a wire rack before cutting. Store in an airtight container.