For the cream layers: Put the cream in a saucepan over medium heat and add the condensed milk, sugar, salt and vanilla bean and seeds. Bring to a simmer, stirring to dissolve the sugar, then turn off the heat and leave to infuse for 10 minutes. Remove the vanilla bean.
Pour half of the infused cream into a second saucepan and add the espresso beans and cocoa powder. Bring to a simmer, whisking to combine, then set to one side for 20 minutes to allow the flavors to infuse. Strain into a jug and return to a clean saucepan.
Divide the powdered gelatin between two bowls. Add 2 tablespoons water to one of the bowls and allow to sit for 5 minutes. Then place the bowl over a saucepan of simmering water and allow the gelatin to melt. Gently whisk the melted gelatin into the vanilla cream, and then pour into a measuring jug. Cover with plastic wrap and chill for about 20 minutes until just beginning to set.
Meanwhile, dissolve the remaining gelatin in 2 tablespoons water, as before, and then whisk into the espresso cream. Pour into a separate measuring jug, cover with plastic wrap and chill for about 20 minutes.
Pour about a quarter of the vanilla cream into six to eight 1-cup glasses or similar and cover with plastic wrap. Stick the glasses in the freezer for about 5 minutes until the layer is just set.
As soon as the vanilla cream is set, pour over a layer of the coffee cream--you decide how thick or thin you want the layers. Return to the freezer for 5 minutes, until set, and then repeat the layers with the rest of the vanilla and espresso cream. (If you find that either mix is setting too quickly in the jugs, just leave them at room temperature.) Refrigerate for at least 1 hour.
To make the final espresso layer: Stir the sugar into the espresso until dissolved. Meanwhile, put the gelatin and 2 tablespoons water into a bowl, set aside for 5 minutes and then set over a saucepan of simmering water. Allow the gelatin to melt, then stir it into the espresso and leave until cool but still pourable. Pour a thin layer over each panna cotta and return to the fridge for another hour or so until set.
Serve the panna cottas on small plates with a spoon and decorate with chocolate-covered espresso beans or a sprinkling of espresso instant powder, if liked.