Line an 8-inch square baking pan with parchment paper and preheat the oven to 325°F (160°C).
To make the crust, combine the oat flour, almond flour, brown sugar, baking powder, and salt in a large bowl. Stir in the melted coconut oil and water until a soft crumble forms. Press firmly into the prepared pan and bake until lightly golden around the edges.
To make the filling, blend the pumpkin puree, coconut milk, and brown sugar until smooth. Add the cornstarch, vanilla extract, salt, and pumpkin pie spice, then blend again until fully combined and pale orange.
Pour the filling over the warm crust and spread into an even layer. Bake until the edges are firm, the center no longer moves when gently shaken, and the surface looks dull rather than wet.
Cool completely in the pan before removing and slicing.
For the glaze, whisk together the powdered sugar, coconut milk, and salt until very thick. Slice the bars first, then drizzle the glaze over the top just before serving.