Preheat oven to 350 degrees. Brush mini muffin tins with softened butter; dust with cocoa. In a medium bowl, whisk together cocoa, flour, baking soda, and salt.
With mixer on medium-high, cream butter and granulated sugar until pale. Add brown sugar; beat until fluffy. Add eggs, one at a time, beating well and scraping bowl. Mix in sour cream. On low, add flour mixture in three batches, alternating with buttermilk, and beginning and ending with flour, until just combined.
Divide batter among tins, filling 3/4 full. Bake until firm to the touch and a cake tester inserted in centers comes out clean, 8 to 10 minutes, rotating halfway through. Transfer tins to racks to cool for 5 minutes. Remove from tins; let cool completely.
To assemble: Halve cupcakes horizontally. Brush bottom (and top for stronger flavor) with coffee syrup. Pipe 2 teaspoons mousse onto each bottom. Dip tops in glaze; place on mousse. Top with coffee beans.
Make the mousse and the glaze just before you're ready to assemble the cupcakes.
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