Preheat oven to 350°F (175°C) and butter a 9x5-inch loaf pan, lining it with parchment. In a large bowl, mash the bananas, then stir in the melted butter, sugar, egg, and vanilla until glossy.

Sprinkle the flour, baking soda, and salt over the wet mixture. Fold together gently with a spatula just until no dry streaks remain.

Scoop a little over half the batter into a second bowl. Stir the prepared matcha paste into the smaller portion until evenly green.

Drop alternating spoonfuls of both batters into the prepared pan. Tap the pan on the counter, then use a butter knife to swirl in a figure-eight motion, two or three times.

Bake for 50–55 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs. Cool in the pan for 15 minutes, then lift out and cool completely on a rack.

Slice thick and serve at room temperature, or toast individual slices for a crisp edge.