Set aside 12 blackberries for garnish (or 6 cut in half, if they are very large). Add the remaining blackberries and 1/4 cup sugar to a medium sauce pot, and bring to a boil. Simmer for 10 minutes, until the berries burst and become jammy.
Strain through a fine-mesh sieve, reserving the blackberry liquid. You should end up with about 1/4-1/2 cup of blackberry syrup. Set the blackberry mash aside to cool.
Preheat the oven to 350 degrees F. Line a 12-cup muffin tin with paper liners. Set aside.
In a large mixing bowl, combine the remaining 1/2 cup sugar, spelt flour, cocoa powder, espresso powder, baking soda, and salt. Stir in the oil, almond milk, and vanilla, and mix until completely combined, but not overmixed. Use a spatula to fold in the blackberry mash, until combined.
Using a cookie scoop, divide the batter between the prepared muffin cups, about 3/4 full. Bake for 18-20 minutes, until a cake-tester inserted into the center of each muffin comes out clean. Remove from the oven and let cool in the pan for 5-10 minutes. Remove from the muffin pan, and let cool completely before frosting.
for the blackberry buttercream
Using a stand up mixer fitted with the paddle attachment, or a hand mixer, beat the butter until light and fluffy. Add the sifted powdered sugar, a half cup or so at a time, until the frosting is thick and creamy. Drizzle in the reserved blackberry syrup until the buttercream frosting reaches your desired consistency, preferably thick enough to hold its shape on the cupcakes.
Use a piping bag with fluted tip – or a butter knife – to frost the cupcakes. Top each with a blackberry. Serve immediately.
Store any leftovers in a covered container at room-temperature or refrigerated if longer than a day.