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For the cupcakes: Preheat oven to 350 degrees and line 2 cupcake sheets with cupcake liners. Measure out the milk in a liquid measuring cup and squeeze in the lemon juice. Set aside for five minutes.
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In a mixing bowl, whisk together sugar, flours, cocoa powder, baking soda, baking powder, and salt. Make a well in the center of the dry ingredients and pour in coconut oil, vanilla extract, other extracts (if using), and lemon milk mixture.
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Fold dry ingredients into wet ingredients until a thick and smooth batter has formed.
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Fill each cupcake liner 3/4 full and bake for 18 minutes.
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Remove from oven and transfer to a wire rack to cool completely before frosting.
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For the icing: In the bowl of a stand mixer with a paddle attachment, whip the butter substitute and powdered sugar on medium speed until completely blended and a thick icing has formed, about three minutes.
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With the mixer running, pour in vanilla extract and beat for another minute.
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Divide icing into two bowl. In the first bowl, whisk in the coffee extract until fully incorporated. In the other bowl, whisk in the zest and grapefruit juice until completely incorporated.
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For the Dulce de Leche: Whisk together coconut milk, coconut sugar, and salt in a medium saucepan or large skillet. Bring to a simmer over medium-high heat and let simmer for 15 minutes or until the mixture has turned into a dark caramel color.
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Remove from heat and transfer to a glass container with a lid to cool.
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To assemble: Divide the frosting evenly between the 24 cupcakes and smooth the frosting along the top with a knife or icing spatula. Drizzle with Dulce de Leche or top with a piece of candied grapefruit and shaved chocolate.