Steam beets using a steamer over a pot with boiling water, until a knife easily pierced the skin. Transfer to a blender and add 1-2 Tbsp. water from the pot. Blend, adding water 1 Tbsp. at a time as needed, until a smooth purée forms. Be careful not to add too much water at a time. You want the purée to be the consistency of applesauce.
Measure out 1 cup purée (you can use any extra purée for another use)
Preheat oven to 350°. Grease a Bundt pan generously with coconut oil.
Whisk almond flour, baking soda, and cocoa powder in a medium bowl; set aside.
Heat chocolate and 2 Tbsp. oil in a medium heatproof bowl set over a saucepan of simmering water, stirring occasionally, until melted. Remove bowl from heat. Stir in vinegar, almond extract, and 1 cup of beet purée until smooth.
Beat eggs, brown sugar, and salt in the large bowl of a stand mixer fitted with the whisk attachment on medium-high speed (or use an electric mixer and large bowl) until more than doubled in volume and mixture holds a ribbon for several seconds when the beater is lifted above batter, about 5–7 minutes. This step is very important because it is key to creating an aerated texture.
Pour the chocolate-beet mixture into egg mixture and beat on low speed until combined. Add in the dry ingredients and Beat on lowest speed, scraping down bowl as needed, until combined.
Transfer batter to prepared pan. Bake cake until the top springs back when gently pressed, 20-30 minutes (depending on size of the pan).
Let cool completely. Carefully flip the cake over onto a serving plate while you make the glaze.
Heat chocolate, oil, and salt in a medium heatproof bowl set over a saucepan of barely simmering water, stirring until chocolate is melted. Let cool, stirring occasionally, until mixture is thickened and cool enough to touch, 10–15 minutes.
Drizzle the glaze over the cake to cover top. Sprinkle lightly with coarse sea salt (optional). Let sit at room temperature until glaze is set.