1Preheat oven to 350 degrees. Line a 12 cup standard sized muffin pan with paper liners.
2Reserve 3 or 4 of the strawberries for garnish. Puree 1 cup of the strawberries in a blender or food processor until smooth. Cut the remaining strawberries into ¼ inch dice. Measure out 1/3 cup of the strawberry puree for the cupcakes and reserve the remaining puree (you should have a couple tablespoons) for the topping.
3In a medium mixing bowl, whisk together the white rice flour, starches, xanthan gum, baking powder, baking soda and salt.
4In a small mixing bowl, stir the sour cream or yogurt with the 1/3 cup strawberry puree and vanilla.
5In an electric mixer on high speed, preferably fitted with a paddle attachment, cream together the butter and 1 cup of the sugar until light and fluffy, about 3 minutes. Turn the mixer to low and add the egg and egg whites, one at a time, mixing well with each addition. Add half the dry ingredients and mix on low until combined, then add the strawberry/sour cream (or yogurt) mixture, mixing until combined and finally mix in the remaining flour mixture. Do not over mix. Remove the mixing bowl from the mixer and with a rubber spatula, scrape the sides and bottom of the mixing bowl making sure all ingredients are well combined. Gently fold in the diced strawberries.
6Divide the batter evenly between the prepared muffin cups, filling each almost to the top. Bake for 20 – 25 minutes or until the tops feel dry and springy and a toothpick inserted in the center of a cupcake comes out clean. Let cool in the pan for 5 minutes then remove to a wire rack to finish cooling.
7Just before serving (or up to a couple hours ahead) whip the heavy cream with the remaining 4 teaspoons of sugar on high until it forms stiff peaks. Gently fold in the reserved strawberry puree leaving it a little streaky.
Dollop or pipe the strawberry whipped cream on the cupcakes. Cut the strawberries reserved for garnish into slices and top each cupcake with two slices, pointy end pointing up, or any other way you desire.