Preheat oven to 350°F (180°C). Line 6 alternating cups of a 12-cup muffin pan with paper or foil liners.

In the work bowl of a stand mixer fitted with the paddle attachment, beat butter and granulated sugar at medium speed until light and fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add egg, and beat until combined.

In a small bowl, whisk together flour, baking powder, and salt. With mixer on low speed, gradually add flour mixture to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating just until combined after each addition, stopping to scrape sides of bowl. Beat in vanilla bean paste until combined. Divide batter among prepared muffin cups (about 3 heaping tablespoons or 80 grams each). Fill empty muffin cups halfway full with water.

Bake until tops are golden brown and a wooden pick inserted in center comes out clean, 25 to 30 minutes. Let cool in pan for 10 minutes. Remove from pan, and let cool completely on a wire rack.

Spread Vanilla Buttercream onto cooled cupcakes as desired. Garnish with sparkling sugar, if desired. Store in an airtight container for up to 3 days.