diff --git a/src/commands/dev/lib/builder-cache.ts b/src/commands/dev/lib/builder-cache.ts
index 28f0740b59d..cce813a73e8 100644
--- a/src/commands/dev/lib/builder-cache.ts
+++ b/src/commands/dev/lib/builder-cache.ts
@@ -119,7 +119,7 @@ export async function cleanCacheDir(output: Output): Promise<void> {
 /**
  * Install a list of builders to the cache directory.
  */
-export async function installBuilders(packagesSet: Set<string>): Promise<void> {
+export async function installBuilders(packagesSet: Set<string>, yarnDir: string): Promise<void> {
   const packages = Array.from(packagesSet);
   if (
     packages.length === 0 || (
@@ -130,6 +130,7 @@ export async function installBuilders(packagesSet: Set<string>): Promise<void> {
     return;
   }
   const cacheDir = await builderDirPromise;
+  const yarnPath = join(yarnDir, 'yarn');
   const buildersPkg = join(cacheDir, 'package.json');
 
   // Pull the same version of `@now/build-utils` that now-cli is using
@@ -140,11 +141,12 @@ export async function installBuilders(packagesSet: Set<string>): Promise<void> {
   );
   try {
     await execa(
-      'npm',
+      process.execPath,
       [
-        'install',
-        '--save-exact',
-        '--no-package-lock',
+        yarnPath,
+        'add',
+        '--exact',
+        '--no-lockfile',
         `${buildUtils}@${buildUtilsVersion}`,
         ...packages.filter(p => p !== '@now/static')
       ],
diff --git a/src/commands/dev/lib/dev-server.ts b/src/commands/dev/lib/dev-server.ts
index f23047a3557..54338b76759 100644
--- a/src/commands/dev/lib/dev-server.ts
+++ b/src/commands/dev/lib/dev-server.ts
@@ -54,7 +54,7 @@ export default class DevServer {
   public env: EnvConfig;
   public buildEnv: EnvConfig;
   public files: BuilderInputs;
-  public yarnPath?: string;
+  public yarnPath: string;
 
   private cachedNowJson: NowConfig | null;
   private server: http.Server;
@@ -71,7 +71,9 @@ export default class DevServer {
     this.env = {};
     this.buildEnv = {};
     this.files = {};
-    this.yarnPath = undefined;
+
+    // This gets updated when `start()` is invoked
+    this.yarnPath = '/';
 
     this.cachedNowJson = null;
     this.server = http.createServer(this.devServerHandler);
@@ -425,7 +427,7 @@ export default class DevServer {
       (nowJson.builds || []).map((b: BuildConfig) => b.use)
     );
 
-    await installBuilders(builders);
+    await installBuilders(builders, this.yarnPath);
     await this.updateBuildMatches(nowJson);
 
     // Now Builders that do not define a `shouldServe()` function need to be
diff --git a/src/commands/dev/lib/yarn-installer.ts b/src/commands/dev/lib/yarn-installer.ts
index 518e68dd9e8..d196945a315 100644
--- a/src/commands/dev/lib/yarn-installer.ts
+++ b/src/commands/dev/lib/yarn-installer.ts
@@ -1,6 +1,5 @@
 import execa from 'execa';
 import { tmpdir } from 'os';
-import which from 'which-promise';
 import { createHash } from 'crypto';
 import {
   mkdirp,
@@ -19,17 +18,6 @@ const YARN_VERSION = '1.15.2';
 const YARN_SHA = '97efd1871117e60c24f157289d61a7595e142070';
 const YARN_URL = `https://github.com/yarnpkg/yarn/releases/download/v${YARN_VERSION}/yarn-${YARN_VERSION}.js`;
 
-async function whichYarn(output: Output): Promise<string | null> {
-  try {
-    const yarnPath = await which('yarn');
-    output.debug(`Found yarn in current $PATH at "${yarnPath}"`);
-    return yarnPath;
-  } catch (error) {
-    output.debug('Did not find yarn in current $PATH');
-    return null;
-  }
-}
-
 function plusxSync(file: string): void {
   const s = statSync(file);
   const newMode = s.mode | 64 | 8 | 1;
@@ -60,7 +48,7 @@ function getSha1(filePath: string): Promise<string | null> {
 
 async function installYarn(output: Output): Promise<string> {
   // Loosely based on https://yarnpkg.com/install.sh
-  const dirName = join(await builderDirPromise, 'node_modules', '.bin');
+  const dirName = await builderDirPromise;
   const yarnBin = join(dirName, 'yarn');
   const sha1 = await getSha1(yarnBin);
 
@@ -91,10 +79,6 @@ async function installYarn(output: Output): Promise<string> {
   return dirName;
 }
 
-export async function getYarnPath(output: Output): Promise<string | undefined> {
-  const path = await whichYarn(output);
-  if (path) {
-    return dirname(path);
-  }
+export async function getYarnPath(output: Output): Promise<string> {
   return installYarn(output);
 }
diff --git a/test/dev-server.unit.js b/test/dev-server.unit.js
index b344aae4f8e..80adfc78302 100644
--- a/test/dev-server.unit.js
+++ b/test/dev-server.unit.js
@@ -57,4 +57,3 @@ test('do not install builders if there are no builds', async t => {
 
   t.pass();
 })
-