CARAMEL FILLING 

1. Add the 2 tablespoons of cold water and the gelatin to a small bowl and mix together.  Let stand until softened, about 5 minutes.  Stir in the 1/4 cup of boiling water until dissolved.  Set aside while you whip the whipping cream.
2. Add the whipping cream to a large mixing bowl.  Using a mixer, beat the whipping cream on medium speed until soft peaks form.  Gradually add the sugar, beating until stiff peaks form.  Add the caramel and beat just until combined.
3. Beat the gelatin mixture into the whipping cream mixture just until combined.  The mixture will be runny, but it will firm up again once refrigerated.
4. Cover and refrigerate for at least 4 hours before using, or up to 2 days. 

CAKE ROLL

1. Preheat oven to 375 degrees.  Lightly butter a 15 x 10-inch rimmed baking sheet and line it with parchment or waxed paper.  Butter the paper and inside edges of the pan and lightly flour.  Alternately, you can use baking spray with flour in it.  Set aside.
2. In a small bowl, whisk together the cake flour, baking powder, and salt.  Set aside.
3. In a medium bowl, using a mixer, beat the egg yolks and 1/3 cup of the sugar on medium-high speed until the mixture is thick and pale in color, about 3 minutes.  Set aside.
4. In a separate large mixing bowl, fitted with a whisk attachment, beat the egg whites at high speed until soft peaks form.  Slowly add the remaining 1/3 cup of sugar and beat until stiff peaks form.
5. Add the egg mixture, pecans, butter, and vanilla to the egg whites and using a spatula gently fold the egg whites into the other ingreients.  Spread the batter into the prepared pan.
6. Bake until the cake springs back when lightly touched in the center or a toothpick comes out clean, about 10 minutes.  Do not overbake.
7. Meanwhile, spread a clean lint-free dish towel, bigger than the baking sheet, onto a counter.  Sift powdered sugar over it. 
8. Remove the cake from the oven and run a knife around the edges of the pan.  Immediately invert the cake onto the towel.  Gently peel off the paper.  Starting with a short end, roll up the cake and towel together.  Place seam side down on a wire rack, and allow to cool completely. 
9. Gently unroll the cooled cake.  Spread a generous amount of the chilled caramel filling on the cake roll, stopping at least 1/4 inch from the edges.  Starting on a short end, slowly reroll the cake with filling, leaving the towel behind as you roll.
10. Place the cake, seam side down, on a serving platter.  Sift powdered sugar over the top, and place pecan halves on the top for garnish, optional.
11. Refrigerate the cake for 2 hours before serving. 
12. To serve, slice the cake into 1-inch thick slices, using a sharp knife, and wiping the knife clean between slices.
13. Cover and refrigerate leftovers for up to 2 days.