Preheat the oven to 350 F, and line a standard cupcake pan with 17 paper liners.
In the bowl of your stand mixer fitted with the paddle attachment, beat the butter, granulated sugar and brown sugar for about 5 minutes, scraping the bowl occasionally, until very light and fluffy.
Add the eggs, one at a time, beating well after each addition, and scraping the bowl down, then add the vanilla.
In a separate bowl, sift together the flour, graham cracker crumbs, baking powder, salt and cinnamon. With the mixer on low, add the dry ingredients in three additions, alternating with the buttermilk, starting and ending with the dry ingredients. Use a large spatula to stir in any remaining streaks of flour.
Use a 1/4 cup measuring cup to spoon the batter into the paper liners, filling them no more than 3/4 full.
Bake the cupcakes on the center oven rack for about 18 minutes, until a toothpick comes out clean. Note that these cupcakes bake very level, and won't have a domed top. Cool completely before frosting.
Buttercream
In the bowl of your stand mixer fitted with the whisk attachment, beat the butter for 1 minute until smooth. (In hot weather, I recommend substituting shortening for half of the butter for better stability.)
With the mixer on low, add the powdered sugar by spoonfuls, the meringue powder and salt, and mix to combine. It will be thick and clumpy.
Add the vanilla, and increase speed to medium (#6 on a Kitchen Aid mixer), and whip for 4-5 minutes, scraping the bowl several times, until light and fluffy. Only add the milk if needed for consistency (usually only necessary in colder weather).
Assembly
Use a cupcake corer or knife to remove a small piece of cake from the center of the cupcakes.
Fit a piping bag with tip 1M, and fill with buttercream. Pipe a "ring" of buttercream around the edge of the cupcake.
Use a spoon (or another piping bag) to fill the centers of the cupcakes with the lime curd.
Sprinkle the tops of the cupcakes with more crushed graham crackers or leftover cake crumbs from removing the centers.
Notes
Homemade Lime Curd: follow this recipe for lemon curd, substituting lime juice and zest instead of lemon.Keep cupcakes refrigerated, in an airtight container, for up to 2 days. Let come to room temperature for about an hour before serving.