Line a rimmed baking sheet with parchment paper, allowing the parchment to extend slightly up the sides for easy removal.
Add the chopped dark chocolate and coconut oil to a microwave-safe bowl. Microwave in 20–30 second intervals, stirring well between each, until completely smooth. Spread about half of the melted chocolate into a thin, even layer on the prepared baking sheet.
Place the baking sheet in the freezer for about 10–15 minutes, or until the chocolate is firm.
Add the dates, peanut butter, and kosher salt to a food processor. Blend until smooth and thick. If the mixture is too thick to blend or spread, add water 1 tablespoon at a time until it reaches a smooth, spreadable consistency. Add honey if desired.
Spread the date and peanut butter mixture evenly over the frozen chocolate layer.
Add the Greek yogurt, banana, honey if using, and vanilla extract to a blender or food processor. Blend until smooth and creamy.
Spread the banana yogurt mixture evenly over the date-peanut butter layer.
Return the baking sheet to the freezer and freeze for about 1–2 hours, or until the yogurt layer is firm.
Re-melt the remaining chocolate mixture if necessary. Spread evenly over the frozen yogurt layer.
Immediately sprinkle the top with flaky sea salt before the chocolate sets.
Return the bark to the freezer for another 30–60 minutes, or until completely firm.
Remove the bark from the freezer and let it sit at room temperature for a few minutes. Cut into pieces with a sharp knife or break into rustic pieces by hand.