Step 1

Preheat oven to 350˚. Pulse oats, whole wheat flour, coconut oil, agave, cocoa, salt, and cinnamon in a food processor until oats are coarsely ground and mixture looks like wet sand.

Step 2

Firmly press into bottom and up the sides of tart pan. Bake until crust is golden brown and smells toasty, 20–25 minutes. Transfer to a wire rack; let cool.

Step 3

Dissolve espresso powder in ¾ cup very hot water in a food processor. Add cocoa and let sit 5 minutes. Add melted chocolate, agave syrup, vanilla, ⅔ cup coconut oil, and ¼ tsp. kosher salt and process until mixture is very smooth and thick, about 30 seconds.

Step 4

Scrape filling into cooled tart shell and smooth top (if making tart in a rectangular shell, you may have a little filling left over; save for another use, like truffles). Tap tart lightly against counter to pop any air bubbles in filling.

Step 5

Cook oats and remaining 1 tsp. coconut oil in a small skillet over low heat, stirring constantly, until oats are toasted, about 3 minutes. Gradually add half of demerara sugar, stirring constantly, until sugar is melted, then remove from heat and stir in remaining demerara sugar just to coat. Season with kosher salt; let oat mixture cool.

Step 6

Scatter oat mixture over tart, sprinkle with sea salt, and chill until filling is set, at least 1 hour.

Step 7

Cut chilled tart into slices with a dry, hot knife; let sit at room temperature 15–20 minutes before serving.

Step 8

Do Ahead: Tart can be made 2 days ahead. Once filling is set, cover and keep chilled.