Preheat oven to 350°.
To prepare cupcakes, combine 2 tablespoons boiling water and 4 teaspoons espresso, stirring until espresso dissolves. Stir in buttermilk.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking soda, and salt, stirring well with a whisk.
Place 3/4 cup granulated sugar, butter, and 2 teaspoons vanilla extract in a large bowl; beat with a mixer at medium speed until well blended (about 5 minutes). Add eggs, 1 at a time, beating well after each addition. Add flour mixture and buttermilk mixture alternately to sugar mixture, beginning and ending with flour mixture.
Spoon batter into 12 muffin cups lined with paper liners. Bake at 350° for 20 minutes or until a wooden pick inserted in center comes out clean. Cool cupcakes in pan 10 minutes on a wire rack; remove from pan.
To prepare syrup, combine 1/4 cup granulated sugar, 1/4 cup water, 2 tablespoons espresso, corn syrup, and 1/4 teaspoon vanilla extract in a small saucepan; bring to a boil. Reduce heat; simmer 3 minutes. Pierce the cupcake tops several times with a wooden skewer. Brush espresso syrup evenly over the cupcakes. Cool completely on wire rack. Sprinkle with powdered sugar.