Cupcakes:

Preheat oven to 180°C (350°F). Line standard muffin tins with paper liners. Whisk dry ingredients in a large bowl. Cream butter and sugar with a mixer until light and fluffy. Add eggs, 1 at a time, beating after each addition.

Reduce speed to low. Mix any remaining wet ingredients in a bowl if needed. Add dry ingredients to butter mixture in 3 additions, alternating with wet ingredients and ending with dry. Scrape sides of bowl. Divide batter among muffin cups, filling each 2/3 full.

Bake cupcakes until testers inserted into centers come out clean, about 20 minutes. Let cool in tins on wire racks.

Strawberry buttercream:

Place whites and sugar in a heatproof mixer bowl set over a pot of simmering water. Whisk until sugar dissolves and mixture registers 160 degrees on a candy thermometer.

Remove from heat, and attach bowl to a mixer. Whisk on medium speed for 5 minutes. Increase speed to medium-high, and whisk until stiff, glossy peaks form, about 6 minutes. Reduce speed to medium, and add butter, 1 piece at a time, whisking well after each addition.

Switch to a paddle attachment. With mixer on low, add strawberry puree, and beat until smooth, 3 to 5 minutes. Use immediately, or cover, and refrigerate for up to 3 days. (Bring to room temperature, and beat on low speed until smooth before using.)