Preheat the oven to 350 degrees F. Line 40 cups from 2 (24-cup) mini cupcake pans or 16 cups from 2 regular-size (12-cup) muffin pans with paper cupcake liners.
Sift or whisk together the flour, sugar, cocoa, baking soda, and salt into a bowl. In a separate bowl, mix the coconut milk, oil, vinegar, vanilla, and espresso powder until smooth. Pour the wet mixture into the dry mixture and mix with a fork or small whisk. Divide the batter evenly among the prepared cupcake liners. The cups should be filled about 2/3-full, and bake until a toothpick inserted in the center comes out clean, about 20 to 22 minutes for the mini cupcakes and about 25 minutes for the regular-size cupcakes. Cool the cupcakes in the pans for 5 minutes and then transfer to a rack to cool completely.
Slice off the top 1/3 of each cupcake and slather with frosting and sliced strawberries. Place the top of the cupcake back on top and add an additional bit of frosting and sliced strawberries. Dust with confectioners sugar, if using.
Combine the confectioners' sugar, margarine, and vanilla extract in the bowl of a stand mixer. Beat on medium-high speed until combined. With the mixer running, add 1 tablespoon of water at a time, until the desired buttercream consistency.