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Preheat the oven to 375°F. Lightly grease a 9" x 13" pan and line it with parchment to overhang the long edges. Clip the ends of the parchment to the sides of the pan.
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To make the base: Whisk together the flour, baking powder, and salt in a medium bowl. In a large mixing bowl, beat together the coconut oil and coconut sugar. Mix in half of the dry ingredients, then the egg and sour cream. Scrape the bowl and mix in the coconut flavor (or extract), vanilla, and remaining dry ingredients.
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Press the batter into the prepared pan. Bake for 14 to 16 minutes, until golden brown. Remove from the oven and cool on a rack.
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To make the filling: Place the chocolate in a heatproof bowl. Bring the cream to a simmer in the microwave or in a small saucepan. Pour the hot cream over the chocolate and let sit for 2 minutes. Stir until the mixture is smooth, then spread over the cooled cookie base in the pan. Refrigerate, uncovered, while you make the frosting.
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To make the frosting: In a medium bowl, beat together the butter, salt, milk, vanilla, and coconut flavor (or extract). Stir in 2 cups of the confectioners' sugar and the coconut milk powder until smooth. Scrape the sides and bottom of the bowl, then stir in the remaining confectioners' sugar and half of the shredded sweetened coconut.
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Spread the frosting over the chilled chocolate layer, then sprinkle the remaining sweetened coconut over the top, pressing it in lightly. Loosen the bars from the edge of the pan on both short sides, and use the parchment to lift them out of the pan and onto a board. Cut in 1 1/2" squares to serve.
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Store any leftovers, well wrapped in plastic, at room temperature for several days; freeze for longer storage.