Preheat oven to 400 degrees F.
Butter 4 mini loaf pans OR 6 mini tart pans OR 1 (12-cup) regular size muffin pan.
Melt unsalted butter. Set aside to cool.
Add egg whites to stand mixer. Beat on high until frothy, about 2 minutes. Remove bowl from stand mixer.
Add granulated sugar and fresh lemon zest to small bowl. Whisk to combine. Add all-purpose flour, almond flour, and sea salt to sugar mixture. Whisk to combine.
Add sugar mixture to egg whites. Fold in using a large spatula until dry ingredients are incorporated.
Add the cooled melted butter to the cake batter. Fold in using a large spatula until smooth and well combined.
Divide the batter evenly between the prepared pans. Place a fresh raspberry or two or three on the top of each cake.
Bake in hot oven for 10 to 12 minutes, or until the edges are golden brown and the cakes are set in the center.
Cool the pans on wire racks for several minutes and then remove the cakes from the pans to cool completely.
Dust with powdered sugar to serve.