To prepare the chocolate syrup, combine the water, allulose, cocoa powder, vanilla extract, and salt in a small skillet set over medium heat. Whisk vigorously until the allulose is dissolved and the cocoa powder is thoroughly incorporated into the mixture, approximately 2-3 minutes. Remove from heat and cool completely. Cover and store in the refrigerator in an airtight container until ready to use. 

Pour heavy whipping cream into a large mixing bowl and beat on high until light and fluffy. Use a spatula while mixing to scrape the sides of the bowl to ensure all of the heavy cream is whipped. Set aside.

Pour keto condensed milk and vanilla extract into a large bowl and stir to combine. If your condensed milk is too thick, thin it by gently folding a small amount of the whipped cream into the bowl. Do not overmix. Add the rest of the whipped cream and fold into the mixture until thoroughly combined.

Pour half the whipped cream mixture into a 1.5-quart ice-cream freezer pan and spread into an even layer. Drizzle half the chocolate syrup and sprinkle some walnuts, if using.

Spoon the remaining whipped cream mixture on top and spread until smooth. Repeat process described in Step 4 with the remaining chocolate syrup and nuts.

Insert a butter knife into the mixture at one end of the loaf pan and pull it back and forth across the length of the pan to incorporate the chocolate and nuts throughout. If using a generic pan, cover with some plastic wrap and gently press across the top to remove any air bubbles. Place in the freezer for a minimum of four hours, or preferably, overnight.

Remove from freezer 10-15 minutes before serving to soften. To serve, scoop into individual bowls and top with additional chocolate sauce and nuts, if desired. Enjoy!