In a saucepan over medium - low heat, combine the butter and Swerve. Bring to a boil, stir occasionally as needed and cook until browned (about 3 minutes). Turn off the heat.
Turn on the heat again and stir over medium - low heat for about 20 - 30 seconds or until it gets thicker.
Preheat the oven to 180 C / 350 F. Line an 8 x 4 inch (20 cm x 10 cm) loaf pan with parchment paper. (* The pan I used is 8.5 x 3.6 inch (22 x 9 cm).)
In a large bowl, beat the butter about one minute at high speed. Add the stevia and beat about two minutes at high speed.
Beat in the eggs in 5 additions, scraping down the sides of the bowl as necessary. Add the vanilla in the last addition. (* Make sure to beat well between each addition (about one minute).)
Transfer 1/3 of the batter (or a bit less than that) to a separate bowl. Add 1/3 cup (80 g) caramel sauce to that bowl and stir well. (* The caramel sauce should crystalize by this stage, so stir over low heat until creamy before adding.)
Add a bit of caramel sauce (about one tsp) to the remaining plain batter and stir well.
Pour the caramel batter into the plain batter bowl and stir lightly (only 4 - 5 times) to create swirls.
Bake for 45 - 50 minutes. (* Cover with aluminum foil halfway through the baking.)
Stir the remaining caramel sauce over low heat until creamy. (* Add the heavy cream if too thick.) Place in a piping bag and pipe over the cooled cake.
Calories 233 kcal, Protein 4.3 g, Total Fat 22.7 g, Total Carbohydrate 3.0 g, Dietary Fiber 1.4 g, Sugar 0.8 g (I didn’t count Swerve as carbs as it doesn’t affect blood sugar levels.)