• Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round or square baking pan.
• In a medium bowl, whisk together the gluten-free flour, baking powder, baking soda, and salt. Set aside.
• In a large bowl, cream the softened butter and sugar-free sweetener until light and fluffy. This should take about 2-3 minutes with an electric mixer.
• Beat in the eggs, one at a time, ensuring each egg is well incorporated before adding the next. Stir in the vanilla extract.
• Gradually add the dry ingredients to the butter mixture, alternating with the sour cream. Begin and end with the dry ingredients. Mix just until combined.
• Gently fold in the chopped strawberries and walnuts (if using).
• Spread the batter evenly into the prepared baking pan. Bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean.
• Allow the cake to cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.

For the Mascarpone Frosting:

• In a medium bowl, beat the mascarpone cheese until smooth.
• Gradually add the sugar-free powdered sweetener and vanilla extract, beating until well combined.
• If the frosting is too thick, add heavy cream one tablespoon at a time until you reach the desired consistency.
• Spread the frosting over the cooled cake and dust with powdered sugar-free sweetener if desired.