Start by preheating your oven to 350 and preparing your cake pan for nonstick.
In a mixing bowl, use a whisk to combine together the flour, baking powder, lemon zest, and sugar. Then make a well in the center of the mixture.
Next, in another bowl, mix together the oat milk, lemon juice, and oil and mix until combined. Once combined, pour these ingredients into the well that you made in the dry ingredients in the first bowl and then mix the ingredients until just combined.
Evenly distribute the cake batter in your prepared cake pan and bake the cake in the center of your oven and bake for 22-35 minutes or until a toothpick comes out mostly clean (a few crumbs are okay) .
Let the cake cool for at least 15 minutes before gently running a butter knife around the edges to loosed it from the pan. Then invert the cake on top of a cooling rack to fully cool.
To make the frosting, beat together the vegan butter, half of the powdered sugar, lemon juice and lemon zest until creamy. Add the remaining sugar and beat until light and fluffy, making adjustments as needed for texture.
Once the cake has fully cooled, use a large bread knife to slice it horizontally in half. It does not have to be exact or perfect as long as you have two halves to make a layered cake with.
On the top side of the bottom cake half, evenly spread the blueberry jam. Then use a spoon to drop dollops of the frosting over the jam layer (using about a quarter of the frosting).
Place the top half of the cake on top of the frosting dollops and use the remaining ingredients to frost and decorate your cake.