Set the eggs out to come to room temperature.
Make the filling! Heat orange juice, orange zest, and sugar in a small saucepan over medium heat until just simmering and the sugar has dissolved. Once the sugar is dissolved, remove from heat and slowly sprinkle the unflavored gelatin powder into the juice while whisking vigorously and continuously. Pour the jelly into an un-greased 9x13-inch glass baking dish. Chill for at least one hour until set and firm to the touch.
Preheat oven to 350 degrees Fahrenheit and grease a 12-hole, shallow bun tin with soft butter.
Beat the eggs and sugar in a large mixing bowl for six minutes, until pale, fluffy, and ribbon stage. (Looks like early whipped cream stage - the batter should ribbon off the beaters.) Sift the flour and baking powder into the bowl and fold into egg mixture.
Spoon two tablespoons of mixture into each bun cup, at least three-quarters full, and smooth the tops. Bake for 7-9 minutes or until well risen and the top of the sponges spring back when lightly pressed. Remove from the oven, let cool in the tin for a few minutes, then remove to a wire rack to cool completely. Repeat with any remaining batter. (*See Notes.)
Remove the jelly from the refrigerator and use a 2-inch biscuit cutter (or jar lid) to cut at least 12 disks out of the jelly. Use a spoon to carefully scoop the jelly disks up from the bottom of the baking dish. (The jelly will be the consistency of very firm gelatin. Scrape the spoon along the bottom of the dish and peel away the excess jelly from the disks.) Place one disk on each sponge cake.
Melt the chocolate chips by placing them in a small bowl and microwaving at 50% power for 30 second intervals. Stir well after each interval--the last of the chocolate bits should melt as you're stirring. Spoon a tablespoon or so of chocolate onto each jaffa cake, using the back of a spoon to push the chocolate to the edge of the jaffa cake, sealing the orange jelly inside. Set the cakes back on a wire rack to let the chocolate set. When the chocolate is almost set, use the tines of a fork to gently dab the criss-cross pattern on the top.
Store leftovers in an airtight container.