Whisk the flour, cocoa powder, baking soda, and kosher salt in a large bowl.
In a separate bowl, whisk the sugar, eggs, melted butter, and vanilla until combined.
Pour the wet ingredients into the dry and mix until a stiff dough forms.
Work in the almonds.
Turn the dough out onto the prepared baking sheet and shape it into a log, roughly 12 inches long and 2 inches wide.
Bake for 25 to 28 minutes until the logs are set, and a few cracks appear on the surface. Remove it from the oven and let it cool on the sheet for 20 minutes. Do not skip this rest or the logs will crumble when sliced.
Reduce the oven to 325 F.
Transfer the logs to a cutting board. Using a sharp serrated knife, cut the log on a slight diagonal into slices ½ inch thick.
Arrange the slices cut side down on the baking sheet in a single layer. Bake for 12 minutes, flip each biscotti, then bake another 12 minutes. The biscotti should feel dry and firm to the touch.
Transfer to a wire rack and cool completely.
Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer. Do not boil. Pour the cream over the chocolate and let it sit for 2 minutes without stirring.
Whisk from the center out, adding more heavy cream if necessary, until the ganache is smooth and glossy. Cool the ganache to room temperature until it’s thick enough to coat without dripping, about 15 minutes.
Dip the bottom half of each biscotti into the ganache, letting the excess drip off.
Place the biscotti on a parchment-lined sheet. Immediately sprinkle the ganache with a pinch of flaky sea salt.
Let the biscotti set at room temperature until the ganache is firm, about 30 minutes, or refrigerate for 10 minutes. Serve.