1In a small saucepan, bring balsamic reduction to a boil over medium high heat. Reduce heat to medium and simmer until vinegar coats the back of a spoon and reduced to about 3/4 cup. Let cool completely. Cover and refrigerate for up to one month.
1Combine the dry ingredients in the bowl of an electric mixer on low speed using the paddle attachment. Increase the speed to medium and add the egg whites in several additions, scraping the bowl between additions. Mix until just combined. Scrape down the sides of the bowl. Slowly add the browned butter until thoroughly incorporated. Fold in the diced strawberries.
2Pour the batter into the muffin tins about two-thirds the way up.
3Drizzle balsamic reduction and swirl with a toothpick.
4Place one strawberry slice on top of each financier and sanding sugar or turbinado sugar to finish.
5Bake until golden brown and the financier is baked through all the way at its centre, about 15 to 18 minutes. Cool slightly, then invert onto cooling rack.