• Preheat oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners.

Make the cupcakes:

• In a large bowl, whisk together the mashed banana, sugar, oil, oat milk, vanilla, and apple cider vinegar until smooth.
• In a separate bowl, whisk the flour, baking powder, baking soda, and salt until well combined.
• Add the dry ingredients to the wet ingredients and stir until just combined and no lumps remain—do not overmix.
• Divide the batter evenly between the muffin cups (about 1/3 cup batter per cup).
• Bake for 19–22 minutes, or until a toothpick inserted into the center comes out clean.
• Let the cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

Make the frosting:

• In a medium bowl, beat the vegan butter and salt with a hand mixer on low speed until creamy.
• Add half the powdered sugar and beat on low until combined. Add the remaining powdered sugar and mix until incorporated.
• Add the peanut butter and beat until smooth. Increase to high speed and whip for 3–5 minutes, until very fluffy.

Make the peanut brittle:

• In a heavy-bottomed saucepan over medium heat, melt the vegan butter and sugar together, swirling occasionally until the sugar is mostly dissolved.
• Reduce heat to medium-low and cook, undisturbed, until the caramel turns a dark golden color. Watch closely—it can burn quickly.
• Remove from the heat, then immediately stir in the salt and baking soda (the mixture will foam slightly).
• Stir in the peanuts until coated, then spread onto a parchment-lined baking sheet in a single layer. Cool completely, then break into small pieces.

Assemble:

• Frost the cooled cupcakes with the peanut butter frosting.
• Top with peanut brittle pieces.
• Serve immediately, or refrigerate in an airtight container for up to 3 days. For longer storage, wrap individually in plastic and freeze for up to 1 month.